# Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

## User's Guide

This section describes how to use xpressive to accomplish text manipulation and parsing tasks. If you are looking for detailed information regarding specific components in xpressive, check the Reference section.

### Introduction

### What is xpressive?

xpressive is a regular expression template library. Regular expressions (regexes) can be written as strings that are parsed dynamically at runtime (dynamic regexes), or as expression templates[9] that are parsed at compile-time (static regexes). Dynamic regexes have the advantage that they can be accepted from the user as input at runtime or read from an initialization file. Static regexes have several advantages. Since they are C++ expressions instead of strings, they can be syntax-checked at compile-time. Also, they can naturally refer to code and data elsewhere in your program, giving you the ability to call back into your code from within a regex match. Finally, since they are statically bound, the compiler can generate faster code for static regexes.

xpressive's dual nature is unique and powerful. Static xpressive is a bit like the Spirit Parser Framework. Like Spirit, you can build grammars with static regexes using expression templates. (Unlike Spirit, xpressive does exhaustive backtracking, trying every possibility to find a match for your pattern.) Dynamic xpressive is a bit like Boost.Regex. In fact, xpressive's interface should be familiar to anyone who has used Boost.Regex. xpressive's innovation comes from allowing you to mix and match static and dynamic regexes in the same program, and even in the same expression! You can embed a dynamic regex in a static regex, or vice versa, and the embedded regex will participate fully in the search, back-tracking as needed to make the match succeed.

### Hello, world!

Enough theory. Let's have a look at Hello World, xpressive style:

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        std::string hello( "hello world!" );

        sregex rex = sregex::compile( "(\\w+) (\\w+)!" );
        smatch what;

        if( regex_match( hello, what, rex ) )
        {
            std::cout << what[0] << '\n'; // whole match
            std::cout << what[1] << '\n'; // first capture
            std::cout << what[2] << '\n'; // second capture
        }

        return 0;
    }

This program outputs the following:

    hello world!
    hello
    world

The first thing you'll notice about the code is that all the types in xpressive live in the `boost::xpressive` namespace.

| Note
Most of the rest of the examples in this document will leave off the `using namespace boost::xpressive;` directive. Just pretend it's there.

Next, you'll notice the type of the regular expression object is `sregex`. If you are familiar with Boost.Regex, this is different than what you are used to. The "`s`" in "`sregex`" stands for "`string`", indicating that this regex can be used to find patterns in `std::string` objects. I'll discuss this difference and its implications in detail later.

Notice how the regex object is initialized:

    sregex rex = sregex::compile( "(\\w+) (\\w+)!" );

To create a regular expression object from a string, you must call a factory method such as ``basic_regex<>::compile()``. This is another area in which xpressive differs from other object-oriented regular expression libraries. Other libraries encourage you to think of a regular expression as a kind of string on steroids. In xpressive, regular expressions are not strings; they are little programs in a domain-specific language. Strings are only one representation of that language. Another representation is an expression template. For example, the above line of code is equivalent to the following:

    sregex rex = (s1= +_w) >> ' ' >> (s2= +_w) >> '!';

This describes the same regular expression, except it uses the domain-specific embedded language defined by static xpressive.

As you can see, static regexes have a syntax that is noticeably different than standard Perl syntax. That is because we are constrained by C++'s syntax. The biggest difference is the use of `>>` to mean "followed by". For instance, in Perl you can just put sub-expressions next to each other:

    abc

But in C++, there must be an operator separating sub-expressions:

    a >> b >> c

In Perl, parentheses `()` have special meaning. They group, but as a side-effect they also create back-references like `$1` and `$2`. In C++, there is no way to overload parentheses to give them side-effects. To get the same effect, we use the special `s1`, `s2`, etc. tokens. Assign to one to create a back-reference (known as a sub-match in xpressive).

You'll also notice that the one-or-more repetition operator `+` has moved from postfix to prefix position. That's because C++ doesn't have a postfix `+` operator. So:

    "\\w+"

is the same as:

    +_w

We'll cover all the other differences later.

### Installing xpressive

### Getting xpressive

There are two ways to get xpressive. The first and simplest is to download the latest version of Boost. Just go to http://sf.net/projects/boost and follow the “Download” link.

The second way is by directly accessing the Boost Subversion repository. Just go to http://svn.boost.org/trac/boost/ and follow the instructions there for anonymous Subversion access. The version in Boost Subversion is unstable.

### Building with xpressive

Xpressive is a header-only template library, which means you don't need to alter your build scripts or link to any separate lib file to use it. All you need to do is `#include <boost/xpressive/xpressive.hpp>`. If you are only using static regexes, you can improve compile times by only including `xpressive_static.hpp`. Likewise, you can include `xpressive_dynamic.hpp` if you only plan on using dynamic regexes.

If you would also like to use semantic actions or custom assertions with your static regexes, you will need to additionally include `regex_actions.hpp`.

### Requirements

Xpressive requires Boost version 1.34.1 or higher.

### Supported Compilers

Currently, Boost.Xpressive is known to work on the following compilers:

- Visual C++ 7.1 and higher
- GNU C++ 3.4 and higher
- Intel for Linux 8.1 and higher
- Intel for Windows 10 and higher
- tru64cxx 71 and higher
- MinGW 3.4 and higher
- HP C/aC++ A.06.14

Check the latest tests results at Boost's Regression Results Page.

| Note
Please send any questions, comments and bug reports to eric <at> boost-consulting <dot> com.

### Quick Start

You don't need to know much to start being productive with xpressive. Let's begin with the nickel tour of the types and algorithms xpressive provides.

Table 39.1. xpressive's Tool-Box

Tool | Description
--- | ---
``basic_regex<>`` | Contains a compiled regular expression. ``basic_regex<>`` is the most important type in xpressive. Everything you do with xpressive will begin with creating an object of type ``basic_regex<>``.
``match_results<>``, ``sub_match<>`` | ``match_results<>`` contains the results of a ``regex_match()`` or ``regex_search()`` operation. It acts like a vector of ``sub_match<>`` objects. A ``sub_match<>`` object contains a marked sub-expression (also known as a back-reference in Perl). It is basically just a pair of iterators representing the begin and end of the marked sub-expression.
``regex_match()`` | Checks to see if a string matches a regex. For ``regex_match()`` to succeed, the whole string must match the regex, from beginning to end. If you give ``regex_match()`` a ``match_results<>``, it will write into it any marked sub-expressions it finds.
``regex_search()`` | Searches a string to find a sub-string that matches the regex. ``regex_search()`` will try to find a match at every position in the string, starting at the beginning, and stopping when it finds a match or when the string is exhausted. As with ``regex_match()``, if you give ``regex_search()`` a ``match_results<>``, it will write into it any marked sub-expressions it finds.
``regex_replace()`` | Given an input string, a regex, and a substitution string, ``regex_replace()`` builds a new string by replacing those parts of the input string that match the regex with the substitution string. The substitution string can contain references to marked sub-expressions.
``regex_iterator<>`` | An STL-compatible iterator that makes it easy to find all the places in a string that match a regex. Dereferencing a ``regex_iterator<>`` returns a ``match_results<>``. Incrementing a ``regex_iterator<>`` finds the next match.
``regex_token_iterator<>`` | Like ``regex_iterator<>``, except dereferencing a ``regex_token_iterator<>`` returns a string. By default, it will return the whole sub-string that the regex matched, but it can be configured to return any or all of the marked sub-expressions one at a time, or even the parts of the string that didn't match the regex.
``regex_compiler<>`` | A factory for ``basic_regex<>`` objects. It "compiles" a string into a regular expression. You will not usually have to deal directly with ``regex_compiler<>`` because the ``basic_regex<>`` class has a factory method that uses ``regex_compiler<>`` internally. But if you need to do anything fancy like create a ``basic_regex<>`` object with a different `std::locale`, you will need to use a ``regex_compiler<>`` explicitly.

Now that you know a bit about the tools xpressive provides, you can pick the right tool for you by answering the following two questions:

1. What iterator type will you use to traverse your data?
2. What do you want to do to your data?

### Know Your Iterator Type

Most of the classes in xpressive are templates that are parameterized on the iterator type. xpressive defines some common typedefs to make the job of choosing the right types easier. You can use the table below to find the right types based on the type of your iterator.

Table 39.2. xpressive Typedefs vs. Iterator Types

| std::string::const_iterator | char const * | std::wstring::const_iterator | wchar_t const *
--- | --- | --- | --- | ---
``basic_regex<>`` | `sregex` | `cregex` | `wsregex` | `wcregex`
``match_results<>`` | `smatch` | `cmatch` | `wsmatch` | `wcmatch`
``regex_compiler<>`` | `sregex_compiler` | `cregex_compiler` | `wsregex_compiler` | `wcregex_compiler`
``regex_iterator<>`` | `sregex_iterator` | `cregex_iterator` | `wsregex_iterator` | `wcregex_iterator`
``regex_token_iterator<>`` | `sregex_token_iterator` | `cregex_token_iterator` | `wsregex_token_iterator` | `wcregex_token_iterator`

You should notice the systematic naming convention. Many of these types are used together, so the naming convention helps you to use them consistently. For instance, if you have a `sregex`, you should also be using a `smatch`.

If you are not using one of those four iterator types, then you can use the templates directly and specify your iterator type.

### Know Your Task

Do you want to find a pattern once? Many times? Search and replace? xpressive has tools for all that and more. Below is a quick reference:

Table 39.3. Tasks and Tools

To do this ... | Use this ...
--- | ---
See if a whole string matches a regex | The ``regex_match()`` algorithm
See if a string contains a sub-string that matches a regex | The ``regex_search()`` algorithm
Replace all sub-strings that match a regex | The ``regex_replace()`` algorithm
Find all the sub-strings that match a regex and step through them one at a time | The ``regex_iterator<>`` class
Split a string into tokens that each match a regex | The ``regex_token_iterator<>`` class
Split a string using a regex as a delimiter | The ``regex_token_iterator<>`` class

These algorithms and classes are described in excruciating detail in the Reference section.

| Tip
Try clicking on a task in the table above to see a complete example program that uses xpressive to solve that particular task.

### Creating a Regex Object

When using xpressive, the first thing you'll do is create a ``basic_regex<>`` object. This section goes over the nuts and bolts of building a regular expression in the two dialects xpressive supports: static and dynamic.

#### Static Regexes

### Overview

The feature that really sets xpressive apart from other C/C++ regular expression libraries is the ability to author a regular expression using C++ expressions. xpressive achieves this through operator overloading, using a technique called expression templates to embed a mini-language dedicated to pattern matching within C++. These "static regexes" have many advantages over their string-based brethren. In particular, static regexes:

- are syntax-checked at compile-time; they will never fail at run-time due to a syntax error.
- can naturally refer to other C++ data and code, including other regexes, making it simple to build grammars out of regular expressions and bind user-defined actions that execute when parts of your regex match.
- are statically bound for better inlining and optimization. Static regexes require no state tables, virtual functions, byte-code or calls through function pointers that cannot be resolved at compile time.
- are not limited to searching for patterns in strings. You can declare a static regex that finds patterns in an array of integers, for instance.

Since we compose static regexes using C++ expressions, we are constrained by the rules for legal C++ expressions. Unfortunately, that means that "classic" regular expression syntax cannot always be mapped cleanly into C++. Rather, we map the regex constructs, picking new syntax that is legal C++.

### Construction and Assignment

You create a static regex by assigning one to an object of type ``basic_regex<>``. For instance, the following defines a regex that can be used to find patterns in objects of type `std::string`:

    sregex re = '$' >> +_d >> '.' >> _d >> _d;

Assignment works similarly.

### Character and String Literals

In static regexes, character and string literals match themselves. For instance, in the regex above, `'$'` and `'.'` match the characters `'$'` and `'.'` respectively. Don't be confused by the fact that `$` and `.` are meta-characters in Perl. In xpressive, literals always represent themselves.

When using literals in static regexes, you must take care that at least one operand is not a literal. For instance, the following are not valid regexes:

    sregex re1 = 'a' >> 'b';         // ERROR!
    sregex re2 = +'a';               // ERROR!

The two operands to the binary `>>` operator are both literals, and the operand of the unary `+` operator is also a literal, so these statements will call the native C++ binary right-shift and unary plus operators, respectively. That's not what we want. To get operator overloading to kick in, at least one operand must be a user-defined type. We can use xpressive's `as_xpr()` helper function to "taint" an expression with regex-ness, forcing operator overloading to find the correct operators. The two regexes above should be written as:

    sregex re1 = as_xpr('a') >> 'b'; // OK
    sregex re2 = +as_xpr('a');       // OK

### Sequencing and Alternation

As you've probably already noticed, sub-expressions in static regexes must be separated by the sequencing operator, `>>`. You can read this operator as "followed by".

    // Match an 'a' followed by a digit
    sregex re = 'a' >> _d;

Alternation works just as it does in Perl with the `|` operator. You can read this operator as "or". For example:

    // match a digit character or a word character one or more times
    sregex re = +( _d | _w );

### Grouping and Captures

In Perl, parentheses `()` have special meaning. They group, but as a side-effect they also create back-references like `$1` and `$2`. In C++, parentheses only group -- there is no way to give them side-effects. To get the same effect, we use the special `s1`, `s2`, etc. tokens. Assigning to one creates a back-reference. You can then use the back-reference later in your expression, like using `\1` and `\2` in Perl. For example, consider the following regex, which finds matching HTML tags:

    "<(\\w+)>.*?</\\1>"

In static xpressive, this would be:

    '<' >> (s1= +_w) >> '>' >> -*_ >> "</" >> s1 >> '>'

Notice how you capture a back-reference by assigning to `s1`, and then you use `s1` later in the pattern to find the matching end tag.

| Tip
Grouping without capturing a back-reference In xpressive, if you just want grouping without capturing a back-reference, you can just use `()` without `s1`. That is the equivalent of Perl's `(?:)` non-capturing grouping construct.

### Case-Insensitivity and Internationalization

Perl lets you make part of your regular expression case-insensitive by using the `(?i:)` pattern modifier. xpressive also has a case-insensitivity pattern modifier, called `icase`. You can use it as follows:

    sregex re = "this" >> icase( "that" );

In this regular expression, `"this"` will be matched exactly, but `"that"` will be matched irrespective of case.

Case-insensitive regular expressions raise the issue of internationalization: how should case-insensitive character comparisons be evaluated? Also, many character classes are locale-specific. Which characters are matched by `digit` and which are matched by `alpha`? The answer depends on the `std::locale` object the regular expression object is using. By default, all regular expression objects use the global locale. You can override the default by using the `imbue()` pattern modifier, as follows:

    std::locale my_locale = /* initialize a std::locale object */;
    sregex re = imbue( my_locale )( +alpha >> +digit );

This regular expression will evaluate `alpha` and `digit` according to `my_locale`. See the section on Localization and Regex Traits for more information about how to customize the behavior of your regexes.

### Static xpressive Syntax Cheat Sheet

The table below lists the familiar regex constructs and their equivalents in static xpressive.

Table 39.4. Perl syntax vs. Static xpressive syntax

Perl | Static xpressive | Meaning
--- | --- | ---
`.` | `_` | any character (assuming Perl's /s modifier).
`ab` | `a >> b` | sequencing of `a` and `b` sub-expressions.
`a|b` | `a | b` | alternation of `a` and `b` sub-expressions.
`(a)` | `(s1= a)` | group and capture a back-reference.
`(?:a)` | `(a)` | group and do not capture a back-reference.
`\1` | `s1` | a previously captured back-reference.
`a*` | `*a` | zero or more times, greedy.
`a+` | `+a` | one or more times, greedy.
`a?` | `!a` | zero or one time, greedy.
`a{n,m}` | `repeat<n,m>(a)` | between `n` and `m` times, greedy.
`a*?` | `-*a` | zero or more times, non-greedy.
`a+?` | `-+a` | one or more times, non-greedy.
`a??` | `-!a` | zero or one time, non-greedy.
`a{n,m}?` | `-repeat<n,m>(a)` | between `n` and `m` times, non-greedy.
`^` | `bos` | beginning of sequence assertion.
`$` | `eos` | end of sequence assertion.
`\b` | `_b` | word boundary assertion.
`\B` | `~_b` | not word boundary assertion.
`\n` | `_n` | literal newline.
`.` | `~_n` | any character except a literal newline (without Perl's /s modifier).
`\r?\n|\r` | `_ln` | logical newline.
`[^\r\n]` | `~_ln` | any single character not a logical newline.
`\w` | `_w` | a word character, equivalent to set[alnum | '_'].
`\W` | `~_w` | not a word character, equivalent to ~set[alnum | '_'].
`\d` | `_d` | a digit character.
`\D` | `~_d` | not a digit character.
`\s` | `_s` | a space character.
`\S` | `~_s` | not a space character.
`[:alnum:]` | `alnum` | an alpha-numeric character.
`[:alpha:]` | `alpha` | an alphabetic character.
`[:blank:]` | `blank` | a horizontal white-space character.
`[:cntrl:]` | `cntrl` | a control character.
`[:digit:]` | `digit` | a digit character.
`[:graph:]` | `graph` | a graphable character.
`[:lower:]` | `lower` | a lower-case character.
`[:print:]` | `print` | a printing character.
`[:punct:]` | `punct` | a punctuation character.
`[:space:]` | `space` | a white-space character.
`[:upper:]` | `upper` | an upper-case character.
`[:xdigit:]` | `xdigit` | a hexadecimal digit character.
`[0-9]` | `range('0','9')` | characters in range `'0'` through `'9'`.
`[abc]` | `as_xpr('a') | 'b' |'c'` | characters `'a'`, `'b'`, or `'c'`.
`[abc]` | `(set= 'a','b','c')` | same as above
`[0-9abc]` | `set[ range('0','9') | 'a' | 'b' | 'c' ]` | characters `'a'`, `'b'`, `'c'` or in range `'0'` through `'9'`.
`[0-9abc]` | `set[ range('0','9') | (set= 'a','b','c') ]` | same as above
`[^abc]` | `~(set= 'a','b','c')` | not characters `'a'`, `'b'`, or `'c'`.
`(?i:stuff)` | `icase(``stuff``)` | match stuff disregarding case.
`(?>stuff)` | `keep(``stuff``)` | independent sub-expression, match stuff and turn off backtracking.
`(?=stuff)` | `before(``stuff``)` | positive look-ahead assertion, match if before stuff but don't include stuff in the match.
`(?!stuff)` | `~before(``stuff``)` | negative look-ahead assertion, match if not before stuff.
`(?<=stuff)` | `after(``stuff``)` | positive look-behind assertion, match if after stuff but don't include stuff in the match. (stuff must be constant-width.)
`(?<!stuff)` | `~after(``stuff``)` | negative look-behind assertion, match if not after stuff. (stuff must be constant-width.)
`(?P<name>stuff)` | ``mark_tag`` `name``(`n`);` ... `(``name``=` `stuff``)` | Create a named capture.
`(?P=name)` | ``mark_tag`` `name``(`n`);` ... `name` | Refer back to a previously created named capture.

#### Dynamic Regexes

### Overview

Static regexes are dandy, but sometimes you need something a bit more ... dynamic. Imagine you are developing a text editor with a regex search/replace feature. You need to accept a regular expression from the end user as input at run-time. There should be a way to parse a string into a regular expression. That's what xpressive's dynamic regexes are for. They are built from the same core components as their static counterparts, but they are late-bound so you can specify them at run-time.

### Construction and Assignment

There are two ways to create a dynamic regex: with the ``basic_regex<>::compile()`` function or with the ``regex_compiler<>`` class template. Use ``basic_regex<>::compile()`` if you want the default locale. Use ``regex_compiler<>`` if you need to specify a different locale. In the section on regex grammars, we'll see another use for ``regex_compiler<>``.

Here is an example of using `basic_regex<>::compile()`:

    sregex re = sregex::compile( "this|that", regex_constants::icase );

Here is the same example using ``regex_compiler<>``:

    sregex_compiler compiler;
    sregex re = compiler.compile( "this|that", regex_constants::icase );

``basic_regex<>::compile()`` is implemented in terms of ``regex_compiler<>``.

### Dynamic xpressive Syntax

Since the dynamic syntax is not constrained by the rules for valid C++ expressions, we are free to use familiar syntax for dynamic regexes. For this reason, the syntax used by xpressive for dynamic regexes follows the lead set by John Maddock's proposal to add regular expressions to the Standard Library. It is essentially the syntax standardized by ECMAScript, with minor changes in support of internationalization.

Since the syntax is documented exhaustively elsewhere, I will simply refer you to the existing standards, rather than duplicate the specification here.

### Internationalization

As with static regexes, dynamic regexes support internationalization by allowing you to specify a different `std::locale`. To do this, you must use ``regex_compiler<>``. The ``regex_compiler<>`` class has an `imbue()` function. After you have imbued a ``regex_compiler<>`` object with a custom `std::locale`, all regex objects compiled by that ``regex_compiler<>`` will use that locale. For example:

    std::locale my_locale = /* initialize your locale object here */;
    sregex_compiler compiler;
    compiler.imbue( my_locale );
    sregex re = compiler.compile( "\\w+|\\d+" );

This regex will use `my_locale` when evaluating the intrinsic character sets `"\\w"` and `"\\d"`.

### Matching and Searching

### Overview

Once you have created a regex object, you can use the ``regex_match()`` and ``regex_search()`` algorithms to find patterns in strings. This page covers the basics of regex matching and searching. In all cases, if you are familiar with how ``regex_match()`` and ``regex_search()`` in the Boost.Regex library work, xpressive's versions work the same way.

### Seeing if a String Matches a Regex

The ``regex_match()`` algorithm checks to see if a regex matches a given input.

| Warning
The ``regex_match()`` algorithm will only report success if the regex matches the whole input, from beginning to end. If the regex matches only a part of the input, ``regex_match()`` will return false. If you want to search through the string looking for sub-strings that the regex matches, use the ``regex_search()`` algorithm.

The input can be a bidirectional range such as `std::string`, a C-style null-terminated string or a pair of iterators. In all cases, the type of the iterator used to traverse the input sequence must match the iterator type used to declare the regex object. (You can use the table in the Quick Start to find the correct regex type for your iterator.)

    cregex cre = +_w;  // this regex can match C-style strings
    sregex sre = +_w;  // this regex can match std::strings

    if( regex_match( "hello", cre ) )              // OK
        { /*...*/ }

    if( regex_match( std::string("hello"), sre ) ) // OK
        { /*...*/ }

    if( regex_match( "hello", sre ) )              // ERROR! iterator mis-match!
        { /*...*/ }

The ``regex_match()`` algorithm optionally accepts a ``match_results<>`` struct as an out parameter. If given, the ``regex_match()`` algorithm fills in the ``match_results<>`` struct with information about which parts of the regex matched which parts of the input.

    cmatch what;
    cregex cre = +(s1= _w);

    // store the results of the regex_match in "what"
    if( regex_match( "hello", what, cre ) )
    {
        std::cout << what[1] << '\n'; // prints "o"
    }

The ``regex_match()`` algorithm also optionally accepts a ``match_flag_type`` bitmask. With ``match_flag_type``, you can control certain aspects of how the match is evaluated. See the ``match_flag_type`` reference for a complete list of the flags and their meanings.

    std::string str("hello");
    sregex sre = bol >> +_w;

    // match_not_bol means that "bol" should not match at [begin,begin)
    if( regex_match( str.begin(), str.end(), sre, regex_constants::match_not_bol ) )
    {
        // should never get here!!!
    }

Click here to see a complete example program that shows how to use ``regex_match()``. And check the ``regex_match()`` reference to see a complete list of the available overloads.

### Searching for Matching Sub-Strings

Use ``regex_search()`` when you want to know if an input sequence contains a sub-sequence that a regex matches. ``regex_search()`` will try to match the regex at the beginning of the input sequence and scan forward in the sequence until it either finds a match or exhausts the sequence.

In all other regards, ``regex_search()`` behaves like ``regex_match()`` (see above). In particular, it can operate on a bidirectional range such as `std::string`, C-style null-terminated strings or iterator ranges. The same care must be taken to ensure that the iterator type of your regex matches the iterator type of your input sequence. As with ``regex_match()``, you can optionally provide a ``match_results<>`` struct to receive the results of the search, and a ``match_flag_type`` bitmask to control how the match is evaluated.

Click here to see a complete example program that shows how to use ``regex_search()``. And check the ``regex_search()`` reference to see a complete list of the available overloads.

### Accessing Results

### Overview

Sometimes, it is not enough to know simply whether a ``regex_match()`` or ``regex_search()`` was successful or not. If you pass an object of type ``match_results<>`` to ``regex_match()`` or ``regex_search()``, then after the algorithm has completed successfully the ``match_results<>`` will contain extra information about which parts of the regex matched which parts of the sequence. In Perl, these sub-sequences are called back-references, and they are stored in the variables `$1`, `$2`, etc. In xpressive, they are objects of type ``sub_match<>``, and they are stored in the ``match_results<>`` structure, which acts as a vector of ``sub_match<>`` objects.

### match_results

So, you've passed a ``match_results<>`` object to a regex algorithm, and the algorithm has succeeded. Now you want to examine the results. Most of what you'll be doing with the ``match_results<>`` object is indexing into it to access its internally stored ``sub_match<>`` objects, but there are a few other things you can do with a ``match_results<>`` object besides.

The table below shows how to access the information stored in a ``match_results<>`` object named `what`.

Table 39.5. match_results<> Accessors

Accessor | Effects
--- | ---
`what.size()` | Returns the number of sub-matches, which is always greater than zero after a successful match because the full match is stored in the zero-th sub-match.
`what[n]` | Returns the n-th sub-match.
`what.length(n)` | Returns the length of the n-th sub-match. Same as `what[n].length()`.
`what.position(n)` | Returns the offset into the input sequence at which the n-th sub-match begins.
`what.str(n)` | Returns a `std::basic_string<>` constructed from the n-th sub-match. Same as `what[n].str()`.
`what.prefix()` | Returns a ``sub_match<>`` object which represents the sub-sequence from the beginning of the input sequence to the start of the full match.
`what.suffix()` | Returns a ``sub_match<>`` object which represents the sub-sequence from the end of the full match to the end of the input sequence.
`what.regex_id()` | Returns the `regex_id` of the ``basic_regex<>`` object that was last used with this ``match_results<>`` object.

There is more you can do with the ``match_results<>`` object, but that will be covered when we talk about Grammars and Nested Matches.

### sub_match

When you index into a ``match_results<>`` object, you get back a ``sub_match<>`` object. A ``sub_match<>`` is basically a pair of iterators. It is defined like this:

    template< class BidirectionalIterator >
    struct sub_match
        : std::pair< BidirectionalIterator, BidirectionalIterator >
    {
        bool matched;
        // ...
    };

Since it inherits publicaly from `std::pair<>`, ``sub_match<>`` has `first` and `second` data members of type `BidirectionalIterator`. These are the beginning and end of the sub-sequence this ``sub_match<>`` represents. ``sub_match<>`` also has a Boolean `matched` data member, which is true if this ``sub_match<>`` participated in the full match.

The following table shows how you might access the information stored in a ``sub_match<>`` object called `sub`.

Table 39.6. sub_match<> Accessors

Accessor | Effects
--- | ---
`sub.length()` | Returns the length of the sub-match. Same as `std::distance(sub.first,sub.second)`.
`sub.str()` | Returns a `std::basic_string<>` constructed from the sub-match. Same as `std::basic_string<char_type>(sub.first,sub.second)`.
`sub.compare(str)` | Performs a string comparison between the sub-match and `str`, where `str` can be a `std::basic_string<>`, C-style null-terminated string, or another sub-match. Same as `sub.str().compare(str)`.

### Results Invalidation

Results are stored as iterators into the input sequence. Anything which invalidates the input sequence will invalidate the match results. For instance, if you match a `std::string` object, the results are only valid until your next call to a non-const member function of that `std::string` object. After that, the results held by the ``match_results<>`` object are invalid. Don't use them!

### String Substitutions

Regular expressions are not only good for searching text; they're good at manipulating it. And one of the most common text manipulation tasks is search-and-replace. xpressive provides the ``regex_replace()`` algorithm for searching and replacing.

### regex_replace()

Performing search-and-replace using ``regex_replace()`` is simple. All you need is an input sequence, a regex object, and a format string or a formatter object. There are several versions of the ``regex_replace()`` algorithm. Some accept the input sequence as a bidirectional container such as `std::string` and returns the result in a new container of the same type. Others accept the input as a null terminated string and return a `std::string`. Still others accept the input sequence as a pair of iterators and writes the result into an output iterator. The substitution may be specified as a string with format sequences or as a formatter object. Below are some simple examples of using string-based substitutions.

    std::string input("This is his face");
    sregex re = as_xpr("his");                // find all occurrences of "his" ...
    std::string format("her");                // ... and replace them with "her"

    // use the version of regex_replace() that operates on strings
    std::string output = regex_replace( input, re, format );
    std::cout << output << '\n';

    // use the version of regex_replace() that operates on iterators
    std::ostream_iterator< char > out_iter( std::cout );
    regex_replace( out_iter, input.begin(), input.end(), re, format );

The above program prints out the following:

    Ther is her face
    Ther is her face

Notice that all the occurrences of `"his"` have been replaced with `"her"`.

Click here to see a complete example program that shows how to use ``regex_replace()``. And check the ``regex_replace()`` reference to see a complete list of the available overloads.

### Replace Options

The ``regex_replace()`` algorithm takes an optional bitmask parameter to control the formatting. The possible values of the bitmask are:

Table 39.7. Format Flags

Flag | Meaning
--- | ---
`format_default` | Recognize the ECMA-262 format sequences (see below).
`format_first_only` | Only replace the first match, not all of them.
`format_no_copy` | Don't copy the parts of the input sequence that didn't match the regex to the output sequence.
`format_literal` | Treat the format string as a literal; that is, don't recognize any escape sequences.
`format_perl` | Recognize the Perl format sequences (see below).
`format_sed` | Recognize the sed format sequences (see below).
`format_all` | In addition to the Perl format sequences, recognize some Boost-specific format sequences.

These flags live in the `xpressive::regex_constants` namespace. If the substitution parameter is a function object instead of a string, the flags `format_literal`, `format_perl`, `format_sed`, and `format_all` are ignored.

### The ECMA-262 Format Sequences

When you haven't specified a substitution string dialect with one of the format flags above, you get the dialect defined by ECMA-262, the standard for ECMAScript. The table below shows the escape sequences recognized in ECMA-262 mode.

Table 39.8. Format Escape Sequences

Escape Sequence | Meaning
--- | ---
`$1`, `$2`, etc. | the corresponding sub-match
`$&` | the full match
`$`` | the match prefix
`$'` | the match suffix
`$$` | a literal `'$'` character

Any other sequence beginning with `'$'` simply represents itself. For example, if the format string were `"$a"` then `"$a"` would be inserted into the output sequence.

### The Sed Format Sequences

When specifying the `format_sed` flag to ``regex_replace()``, the following escape sequences are recognized:

Table 39.9. Sed Format Escape Sequences

Escape Sequence | Meaning
--- | ---
`\1`, `\2`, etc. | The corresponding sub-match
`&` | the full match
`\a` | A literal `'\a'`
`\e` | A literal `char_type(27)`
`\f` | A literal `'\f'`
`\n` | A literal `'\n'`
`\r` | A literal `'\r'`
`\t` | A literal `'\t'`
`\v` | A literal `'\v'`
`\xFF` | A literal `char_type(0xFF)`, where `F` is any hex digit
`\x{FFFF}` | A literal `char_type(0xFFFF)`, where `F` is any hex digit
`\cX` | The control character `X`

### The Perl Format Sequences

When specifying the `format_perl` flag to ``regex_replace()``, the following escape sequences are recognized:

Table 39.10. Perl Format Escape Sequences

Escape Sequence | Meaning
--- | ---
`$1`, `$2`, etc. | the corresponding sub-match
`$&` | the full match
`$`` | the match prefix
`$'` | the match suffix
`$$` | a literal `'$'` character
`\a` | A literal `'\a'`
`\e` | A literal `char_type(27)`
`\f` | A literal `'\f'`
`\n` | A literal `'\n'`
`\r` | A literal `'\r'`
`\t` | A literal `'\t'`
`\v` | A literal `'\v'`
`\xFF` | A literal `char_type(0xFF)`, where `F` is any hex digit
`\x{FFFF}` | A literal `char_type(0xFFFF)`, where `F` is any hex digit
`\cX` | The control character `X`
`\l` | Make the next character lowercase
`\L` | Make the rest of the substitution lowercase until the next `\E`
`\u` | Make the next character uppercase
`\U` | Make the rest of the substitution uppercase until the next `\E`
`\E` | Terminate `\L` or `\U`
`\1`, `\2`, etc. | The corresponding sub-match
`\g<name>` | The named backref name

### The Boost-Specific Format Sequences

When specifying the `format_all` flag to ``regex_replace()``, the escape sequences recognized are the same as those above for `format_perl`. In addition, conditional expressions of the following form are recognized:

    ?Ntrue-expression:false-expression

where N is a decimal digit representing a sub-match. If the corresponding sub-match participated in the full match, then the substitution is true-expression. Otherwise, it is false-expression. In this mode, you can use parens `()` for grouping. If you want a literal paren, you must escape it as `\(`.

### Formatter Objects

Format strings are not always expressive enough for all your text substitution needs. Consider the simple example of wanting to map input strings to output strings, as you may want to do with environment variables. Rather than a format string, for this you would use a formatter object. Consider the following code, which finds embedded environment variables of the form `"$(XYZ)"` and computes the substitution string by looking up the environment variable in a map.

    #include <map>
    #include <string>
    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>
    using namespace boost;
    using namespace xpressive;

    std::map<std::string, std::string> env;

    std::string const &format_fun(smatch const &what)
    {
        return env[what[1].str()];
    }

    int main()
    {
        env["X"] = "this";
        env["Y"] = "that";

        std::string input("\"$(X)\" has the value \"$(Y)\"");

        // replace strings like "$(XYZ)" with the result of env["XYZ"]
        sregex envar = "$(" >> (s1 = +_w) >> ')';
        std::string output = regex_replace(input, envar, format_fun);
        std::cout << output << std::endl;
    }

In this case, we use a function, `format_fun()` to compute the substitution string on the fly. It accepts a ``match_results<>`` object which contains the results of the current match. `format_fun()` uses the first submatch as a key into the global `env` map. The above code displays:

    "this" has the value "that"

The formatter need not be an ordinary function. It may be an object of class type. And rather than return a string, it may accept an output iterator into which it writes the substitution. Consider the following, which is functionally equivalent to the above.

    #include <map>
    #include <string>
    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>
    using namespace boost;
    using namespace xpressive;

    struct formatter
    {
        typedef std::map<std::string, std::string> env_map;
        env_map env;

        template<typename Out>
        Out operator()(smatch const &what, Out out) const
        {
            env_map::const_iterator where = env.find(what[1]);
            if(where != env.end())
            {
                std::string const &sub = where->second;
                out = std::copy(sub.begin(), sub.end(), out);
            }
            return out;
        }

    };

    int main()
    {
        formatter fmt;
        fmt.env["X"] = "this";
        fmt.env["Y"] = "that";

        std::string input("\"$(X)\" has the value \"$(Y)\"");

        sregex envar = "$(" >> (s1 = +_w) >> ')';
        std::string output = regex_replace(input, envar, fmt);
        std::cout << output << std::endl;
    }

The formatter must be a callable object -- a function or a function object -- that has one of three possible signatures, detailed in the table below. For the table, `fmt` is a function pointer or function object, `what` is a ``match_results<>`` object, `out` is an OutputIterator, and `flags` is a value of `regex_constants::match_flag_type`:

Table 39.11. Formatter Signatures

Formatter Invocation | Return Type | Semantics
--- | --- | ---
`fmt(what)` | Range of characters (e.g. `std::string`) or null-terminated string | The string matched by the regex is replaced with the string returned by the formatter.
`fmt(what, out)` | OutputIterator | The formatter writes the replacement string into `out` and returns `out`.
`fmt(what, out, flags)` | OutputIterator | The formatter writes the replacement string into `out` and returns `out`. The `flags` parameter is the value of the match flags passed to the ``regex_replace()`` algorithm.

### Formatter Expressions

In addition to format strings and formatter objects, ``regex_replace()`` also accepts formatter expressions. A formatter expression is a lambda expression that generates a string. It uses the same syntax as that for Semantic Actions, which are covered later. The above example, which uses ``regex_replace()`` to substitute strings for environment variables, is repeated here using a formatter expression.

    #include <map>
    #include <string>
    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>
    #include <boost/xpressive/regex_actions.hpp>
    using namespace boost::xpressive;

    int main()
    {
        std::map<std::string, std::string> env;
        env["X"] = "this";
        env["Y"] = "that";

        std::string input("\"$(X)\" has the value \"$(Y)\"");

        sregex envar = "$(" >> (s1 = +_w) >> ')';
        std::string output = regex_replace(input, envar, ref(env)[s1]);
        std::cout << output << std::endl;
    }

In the above, the formatter expression is `ref(env)[s1]`. This means to use the value of the first submatch, `s1`, as a key into the `env` map. The purpose of `xpressive::ref()` here is to make the reference to the `env` local variable lazy so that the index operation is deferred until we know what to replace `s1` with.

### String Splitting and Tokenization

``regex_token_iterator<>`` is the Ginsu knife of the text manipulation world. It slices! It dices! This section describes how to use the highly-configurable ``regex_token_iterator<>`` to chop up input sequences.

### Overview

You initialize a ``regex_token_iterator<>`` with an input sequence, a regex, and some optional configuration parameters. The ``regex_token_iterator<>`` will use ``regex_search()`` to find the first place in the sequence that the regex matches. When dereferenced, the ``regex_token_iterator<>`` returns a token in the form of a `std::basic_string<>`. Which string it returns depends on the configuration parameters. By default it returns a string corresponding to the full match, but it could also return a string corresponding to a particular marked sub-expression, or even the part of the sequence that didn't match. When you increment the ``regex_token_iterator<>``, it will move to the next token. Which token is next depends on the configuration parameters. It could simply be a different marked sub-expression in the current match, or it could be part or all of the next match. Or it could be the part that didn't match.

As you can see, ``regex_token_iterator<>`` can do a lot. That makes it hard to describe, but some examples should make it clear.

### Example 1: Simple Tokenization

This example uses ``regex_token_iterator<>`` to chop a sequence into a series of tokens consisting of words.

    std::string input("This is his face");
    sregex re = +_w;                      // find a word

    // iterate over all the words in the input
    sregex_token_iterator begin( input.begin(), input.end(), re ), end;

    // write all the words to std::cout
    std::ostream_iterator< std::string > out_iter( std::cout, "\n" );
    std::copy( begin, end, out_iter );

This program displays the following:

    This
    is
    his
    face

### Example 2: Simple Tokenization, Reloaded

This example also uses ``regex_token_iterator<>`` to chop a sequence into a series of tokens consisting of words, but it uses the regex as a delimiter. When we pass a `-1` as the last parameter to the ``regex_token_iterator<>`` constructor, it instructs the token iterator to consider as tokens those parts of the input that didn't match the regex.

    std::string input("This is his face");
    sregex re = +_s;                      // find white space

    // iterate over all non-white space in the input. Note the -1 below:
    sregex_token_iterator begin( input.begin(), input.end(), re, -1 ), end;

    // write all the words to std::cout
    std::ostream_iterator< std::string > out_iter( std::cout, "\n" );
    std::copy( begin, end, out_iter );

This program displays the following:

    This
    is
    his
    face

### Example 3: Simple Tokenization, Revolutions

This example also uses ``regex_token_iterator<>`` to chop a sequence containing a bunch of dates into a series of tokens consisting of just the years. When we pass a positive integer `N` as the last parameter to the ``regex_token_iterator<>`` constructor, it instructs the token iterator to consider as tokens only the `N`-th marked sub-expression of each match.

    std::string input("01/02/2003 blahblah 04/23/1999 blahblah 11/13/1981");
    sregex re = sregex::compile("(\\d{2})/(\\d{2})/(\\d{4})"); // find a date

    // iterate over all the years in the input. Note the 3 below, corresponding to the 3rd sub-expression:
    sregex_token_iterator begin( input.begin(), input.end(), re, 3 ), end;

    // write all the words to std::cout
    std::ostream_iterator< std::string > out_iter( std::cout, "\n" );
    std::copy( begin, end, out_iter );

This program displays the following:

    2003
    1999
    1981

### Example 4: Not-So-Simple Tokenization

This example is like the previous one, except that instead of tokenizing just the years, this program turns the days, months and years into tokens. When we pass an array of integers `{I,J,...}` as the last parameter to the ``regex_token_iterator<>`` constructor, it instructs the token iterator to consider as tokens the `I`-th, `J`-th, etc. marked sub-expression of each match.

    std::string input("01/02/2003 blahblah 04/23/1999 blahblah 11/13/1981");
    sregex re = sregex::compile("(\\d{2})/(\\d{2})/(\\d{4})"); // find a date

    // iterate over the days, months and years in the input
    int const sub_matches[] = { 2, 1, 3 }; // day, month, year
    sregex_token_iterator begin( input.begin(), input.end(), re, sub_matches ), end;

    // write all the words to std::cout
    std::ostream_iterator< std::string > out_iter( std::cout, "\n" );
    std::copy( begin, end, out_iter );

This program displays the following:

    02
    01
    2003
    23
    04
    1999
    13
    11
    1981

The `sub_matches` array instructs the ``regex_token_iterator<>`` to first take the value of the 2nd sub-match, then the 1st sub-match, and finally the 3rd. Incrementing the iterator again instructs it to use ``regex_search()`` again to find the next match. At that point, the process repeats -- the token iterator takes the value of the 2nd sub-match, then the 1st, et cetera.

### Named Captures

### Overview

For complicated regular expressions, dealing with numbered captures can be a pain. Counting left parentheses to figure out which capture to reference is no fun. Less fun is the fact that merely editing a regular expression could cause a capture to be assigned a new number, invaliding code that refers back to it by the old number.

Other regular expression engines solve this problem with a feature called named captures. This feature allows you to assign a name to a capture, and to refer back to the capture by name rather by number. Xpressive also supports named captures, both in dynamic and in static regexes.

### Dynamic Named Captures

For dynamic regular expressions, xpressive follows the lead of other popular regex engines with the syntax of named captures. You can create a named capture with `"(?P<xxx>...)"` and refer back to that capture with `"(?P=xxx)"`. Here, for instance, is a regular expression that creates a named capture and refers back to it:

    // Create a named capture called "char" that matches a single
    // character and refer back to that capture by name.
    sregex rx = sregex::compile("(?P<char>.)(?P=char)");

The effect of the above regular expression is to find the first doubled character.

Once you have executed a match or search operation using a regex with named captures, you can access the named capture through the ``match_results<>`` object using the capture's name.

    std::string str("tweet");
    sregex rx = sregex::compile("(?P<char>.)(?P=char)");
    smatch what;
    if(regex_search(str, what, rx))
    {
        std::cout << "char = " << what["char"] << std::endl;
    }

The above code displays:

    char = e

You can also refer back to a named capture from within a substitution string. The syntax for that is `"\\g<xxx>"`. Below is some code that demonstrates how to use named captures when doing string substitution.

    std::string str("tweet");
    sregex rx = sregex::compile("(?P<char>.)(?P=char)");
    str = regex_replace(str, rx, "**\\g<char>**", regex_constants::format_perl);
    std::cout << str << std::endl;

Notice that you have to specify `format_perl` when using named captures. Only the perl syntax recognizes the `"\\g<xxx>"` syntax. The above code displays:

    tw**e**t

### Static Named Captures

If you're using static regular expressions, creating and using named captures is even easier. You can use the ``mark_tag`` type to create a variable that you can use like `s1`, `s2` and friends, but with a name that is more meaningful. Below is how the above example would look using static regexes:

    mark_tag char_(1); // char_ is now a synonym for s1
    sregex rx = (char_= _) >> char_;

After a match operation, you can use the `mark_tag` to index into the ``match_results<>`` to access the named capture:

    std::string str("tweet");
    mark_tag char_(1);
    sregex rx = (char_= _) >> char_;
    smatch what;
    if(regex_search(str, what, rx))
    {
        std::cout << what[char_] << std::endl;
    }

The above code displays:

    char = e

When doing string substitutions with ``regex_replace()``, you can use named captures to create format expressions as below:

    std::string str("tweet");
    mark_tag char_(1);
    sregex rx = (char_= _) >> char_;
    str = regex_replace(str, rx, "**" + char_ + "**");
    std::cout << str << std::endl;

The above code displays:

    tw**e**t

| Note
You need to include `<boost/xpressive/regex_actions.hpp>` to use format expressions.

### Grammars and Nested Matches

### Overview

One of the key benefits of representing regexes as C++ expressions is the ability to easily refer to other C++ code and data from within the regex. This enables programming idioms that are not possible with other regular expression libraries. Of particular note is the ability for one regex to refer to another regex, allowing you to build grammars out of regular expressions. This section describes how to embed one regex in another by value and by reference, how regex objects behave when they refer to other regexes, and how to access the tree of results after a successful parse.

### Embedding a Regex by Value

The ``basic_regex<>`` object has value semantics. When a regex object appears on the right-hand side in the definition of another regex, it is as if the regex were embedded by value; that is, a copy of the nested regex is stored by the enclosing regex. The inner regex is invoked by the outer regex during pattern matching. The inner regex participates fully in the match, back-tracking as needed to make the match succeed.

Consider a text editor that has a regex-find feature with a whole-word option. You can implement this with xpressive as follows:

    find_dialog dlg;
    if( dialog_ok == dlg.do_modal() )
    {
        std::string pattern = dlg.get_text();          // the pattern the user entered
        bool whole_word = dlg.whole_word.is_checked(); // did the user select the whole-word option?

        sregex re = sregex::compile( pattern );        // try to compile the pattern

        if( whole_word )
        {
            // wrap the regex in begin-word / end-word assertions
            re = bow >> re >> eow;
        }

        // ... use re ...
    }

Look closely at this line:

    // wrap the regex in begin-word / end-word assertions
    re = bow >> re >> eow;

This line creates a new regex that embeds the old regex by value. Then, the new regex is assigned back to the original regex. Since a copy of the old regex was made on the right-hand side, this works as you might expect: the new regex has the behavior of the old regex wrapped in begin- and end-word assertions.

| Note
Note that `re = bow >> re >> eow` does not define a recursive regular expression, since regex objects embed by value by default. The next section shows how to define a recursive regular expression by embedding a regex by reference.

### Embedding a Regex by Reference

If you want to be able to build recursive regular expressions and context-free grammars, embedding a regex by value is not enough. You need to be able to make your regular expressions self-referential. Most regular expression engines don't give you that power, but xpressive does.

| Tip
The theoretical computer scientists out there will correctly point out that a self-referential regular expression is not "regular", so in the strict sense, xpressive isn't really a regular expression engine at all. But as Larry Wall once said, "the term [regular expression] has grown with the capabilities of our pattern matching engines, so I'm not going to try to fight linguistic necessity here."

Consider the following code, which uses the `by_ref()` helper to define a recursive regular expression that matches balanced, nested parentheses:

    sregex parentheses;
    parentheses                          // A balanced set of parentheses ...
        = '('                            // is an opening parenthesis ...
            >>                           // followed by ...
             *(                          // zero or more ...
                keep( +~(set='(',')') )  // of a bunch of things that are not parentheses ...
              |                          // or ...
                by_ref(parentheses)      // a balanced set of parentheses
              )                          //   (ooh, recursion!) ...
            >>                           // followed by ...
          ')'                            // a closing parenthesis
        ;

Matching balanced, nested tags is an important text processing task, and it is one that "classic" regular expressions cannot do. The `by_ref()` helper makes it possible. It allows one regex object to be embedded in another by reference. Since the right-hand side holds `parentheses` by reference, assigning the right-hand side back to `parentheses` creates a cycle, which will execute recursively.

### Building a Grammar

Once we allow self-reference in our regular expressions, the genie is out of the bottle and all manner of fun things are possible. In particular, we can now build grammars out of regular expressions. Let's have a look at the text-book grammar example: the humble calculator.

    sregex group, factor, term, expression;

    group       = '(' >> by_ref(expression) >> ')';
    factor      = +_d | group;
    term        = factor >> *(('*' >> factor) | ('/' >> factor));
    expression  = term >> *(('+' >> term) | ('-' >> term));

The regex `expression` defined above does something rather remarkable for a regular expression: it matches mathematical expressions. For example, if the input string were `"foo 9*(10+3) bar"`, this pattern would match `"9*(10+3)"`. It only matches well-formed mathematical expressions, where the parentheses are balanced and the infix operators have two arguments each. Don't try this with just any regular expression engine!

Let's take a closer look at this regular expression grammar. Notice that it is cyclic: `expression` is implemented in terms of `term`, which is implemented in terms of `factor`, which is implemented in terms of `group`, which is implemented in terms of `expression`, closing the loop. In general, the way to define a cyclic grammar is to forward-declare the regex objects and embed by reference those regular expressions that have not yet been initialized. In the above grammar, there is only one place where we need to reference a regex object that has not yet been initialized: the definition of `group`. In that place, we use `by_ref()` to embed `expression` by reference. In all other places, it is sufficient to embed the other regex objects by value, since they have already been initialized and their values will not change.

| Tip
Embed by value if possible In general, prefer embedding regular expressions by value rather than by reference. It involves one less indirection, making your patterns match a little faster. Besides, value semantics are simpler and will make your grammars easier to reason about. Don't worry about the expense of "copying" a regex. Each regex object shares its implementation with all of its copies.

### Dynamic Regex Grammars

Using ``regex_compiler<>``, you can also build grammars out of dynamic regular expressions. You do that by creating named regexes, and referring to other regexes by name. Each ``regex_compiler<>`` instance keeps a mapping from names to regexes that have been created with it.

You can create a named dynamic regex by prefacing your regex with `"(?$name=)"`, where name is the name of the regex. You can refer to a named regex from another regex with `"(?$name)"`. The named regex does not need to exist yet at the time it is referenced in another regex, but it must exist by the time you use the regex.

Below is a code fragment that uses dynamic regex grammars to implement the calculator example from above.

    using namespace boost::xpressive;
    using namespace regex_constants;

    sregex expr;

    {
         sregex_compiler compiler;
         syntax_option_type x = ignore_white_space;

                compiler.compile("(? $group  = ) \\( (? $expr ) \\) ", x);
                compiler.compile("(? $factor = ) \\d+ | (? $group ) ", x);
                compiler.compile("(? $term   = ) (? $factor )"
                                 " ( \\* (? $factor ) | / (? $factor ) )* ", x);
         expr = compiler.compile("(? $expr   = ) (? $term )"
                                 "   ( \\+ (? $term ) | - (? $term )   )* ", x);
    }

    std::string str("foo 9*(10+3) bar");
    smatch what;

    if(regex_search(str, what, expr))
    {
         // This prints "9*(10+3)":
         std::cout << what[0] << std::endl;
    }

As with static regex grammars, nested regex invocations create nested match results (see Nested Results below). The result is a complete parse tree for string that matched. Unlike static regexes, dynamic regexes are always embedded by reference, not by value.

### Cyclic Patterns, Copying and Memory Management, Oh My!

The calculator examples above raises a number of very complicated memory-management issues. Each of the four regex objects refer to each other, some directly and some indirectly, some by value and some by reference. What if we were to return one of them from a function and let the others go out of scope? What becomes of the references? The answer is that the regex objects are internally reference counted, such that they keep their referenced regex objects alive as long as they need them. So passing a regex object by value is never a problem, even if it refers to other regex objects that have gone out of scope.

Those of you who have dealt with reference counting are probably familiar with its Achilles Heel: cyclic references. If regex objects are reference counted, what happens to cycles like the one created in the calculator examples? Are they leaked? The answer is no, they are not leaked. The ``basic_regex<>`` object has some tricky reference tracking code that ensures that even cyclic regex grammars are cleaned up when the last external reference goes away. So don't worry about it. Create cyclic grammars, pass your regex objects around and copy them all you want. It is fast and efficient and guaranteed not to leak or result in dangling references.

### Nested Regexes and Sub-Match Scoping

Nested regular expressions raise the issue of sub-match scoping. If both the inner and outer regex write to and read from the same sub-match vector, chaos would ensue. The inner regex would stomp on the sub-matches written by the outer regex. For example, what does this do?

    sregex inner = sregex::compile( "(.)\\1" );
    sregex outer = (s1= _) >> inner >> s1;

The author probably didn't intend for the inner regex to overwrite the sub-match written by the outer regex. The problem is particularly acute when the inner regex is accepted from the user as input. The author has no way of knowing whether the inner regex will stomp the sub-match vector or not. This is clearly not acceptable.

Instead, what actually happens is that each invocation of a nested regex gets its own scope. Sub-matches belong to that scope. That is, each nested regex invocation gets its own copy of the sub-match vector to play with, so there is no way for an inner regex to stomp on the sub-matches of an outer regex. So, for example, the regex `outer` defined above would match `"ABBA"`, as it should.

### Nested Results

If nested regexes have their own sub-matches, there should be a way to access them after a successful match. In fact, there is. After a ``regex_match()`` or ``regex_search()``, the ``match_results<>`` struct behaves like the head of a tree of nested results. The ``match_results<>`` class provides a `nested_results()` member function that returns an ordered sequence of ``match_results<>`` structures, representing the results of the nested regexes. The order of the nested results is the same as the order in which the nested regex objects matched.

Take as an example the regex for balanced, nested parentheses we saw earlier:

    sregex parentheses;
    parentheses = '(' >> *( keep( +~(set='(',')') ) | by_ref(parentheses) ) >> ')';

    smatch what;
    std::string str( "blah blah( a(b)c (c(e)f (g)h )i (j)6 )blah" );

    if( regex_search( str, what, parentheses ) )
    {
        // display the whole match
        std::cout << what[0] << '\n';

        // display the nested results
        std::for_each(
            what.nested_results().begin(),
            what.nested_results().end(),
            output_nested_results() );
    }

This program displays the following:

    ( a(b)c (c(e)f (g)h )i (j)6 )
        (b)
        (c(e)f (g)h )
            (e)
            (g)
        (j)

Here you can see how the results are nested and that they are stored in the order in which they are found.

| Tip
See the definition of output_nested_results in the Examples section.

### Filtering Nested Results

Sometimes a regex will have several nested regex objects, and you want to know which result corresponds to which regex object. That's where `basic_regex<>::regex_id()` and `match_results<>::regex_id()` come in handy. When iterating over the nested results, you can compare the regex id from the results to the id of the regex object you're interested in.

To make this a bit easier, xpressive provides a predicate to make it simple to iterate over just the results that correspond to a certain nested regex. It is called `regex_id_filter_predicate`, and it is intended to be used with Boost.Iterator. You can use it as follows:

    sregex name = +alpha;
    sregex integer = +_d;
    sregex re = *( *_s >> ( name | integer ) );

    smatch what;
    std::string str( "marsha 123 jan 456 cindy 789" );

    if( regex_match( str, what, re ) )
    {
        smatch::nested_results_type::const_iterator begin = what.nested_results().begin();
        smatch::nested_results_type::const_iterator end   = what.nested_results().end();

        // declare filter predicates to select just the names or the integers
        sregex_id_filter_predicate name_id( name.regex_id() );
        sregex_id_filter_predicate integer_id( integer.regex_id() );

        // iterate over only the results from the name regex
        std::for_each(
            boost::make_filter_iterator( name_id, begin, end ),
            boost::make_filter_iterator( name_id, end, end ),
            output_result
            );

        std::cout << '\n';

        // iterate over only the results from the integer regex
        std::for_each(
            boost::make_filter_iterator( integer_id, begin, end ),
            boost::make_filter_iterator( integer_id, end, end ),
            output_result
            );
    }

where `output_results` is a simple function that takes a `smatch` and displays the full match. Notice how we use the `regex_id_filter_predicate` together with `basic_regex<>::regex_id()` and `boost::make_filter_iterator()` from the Boost.Iterator to select only those results corresponding to a particular nested regex. This program displays the following:

    marsha
    jan
    cindy
    123
    456
    789

### Semantic Actions and User-Defined Assertions

### Overview

Imagine you want to parse an input string and build a `std::map<>` from it. For something like that, matching a regular expression isn't enough. You want to do something when parts of your regular expression match. Xpressive lets you attach semantic actions to parts of your static regular expressions. This section shows you how.

### Semantic Actions

Consider the following code, which uses xpressive's semantic actions to parse a string of word/integer pairs and stuffs them into a `std::map<>`. It is described below.

    #include <string>
    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>
    #include <boost/xpressive/regex_actions.hpp>
    using namespace boost::xpressive;

    int main()
    {
        std::map<std::string, int> result;
        std::string str("aaa=>1 bbb=>23 ccc=>456");

        // Match a word and an integer, separated by =>,
        // and then stuff the result into a std::map<>
        sregex pair = ( (s1= +_w) >> "=>" >> (s2= +_d) )
            [ ref(result)[s1] = as<int>(s2) ];

        // Match one or more word/integer pairs, separated
        // by whitespace.
        sregex rx = pair >> *(+_s >> pair);

        if(regex_match(str, rx))
        {
            std::cout << result["aaa"] << '\n';
            std::cout << result["bbb"] << '\n';
            std::cout << result["ccc"] << '\n';
        }

        return 0;
    }

This program prints the following:

    1
    23
    456

The regular expression `pair` has two parts: the pattern and the action. The pattern says to match a word, capturing it in sub-match 1, and an integer, capturing it in sub-match 2, separated by `"=>"`. The action is the part in square brackets: `[ ref(result)[s1] = as<int>(s2) ]`. It says to take sub-match one and use it to index into the `results` map, and assign to it the result of converting sub-match 2 to an integer.

| Note
To use semantic actions with your static regexes, you must `#include <boost/xpressive/regex_actions.hpp>`

How does this work? Just as the rest of the static regular expression, the part between brackets is an expression template. It encodes the action and executes it later. The expression `ref(result)` creates a lazy reference to the `result` object. The larger expression `ref(result)[s1]` is a lazy map index operation. Later, when this action is getting executed, `s1` gets replaced with the first ``sub_match<>``. Likewise, when `as<int>(s2)` gets executed, `s2` is replaced with the second ``sub_match<>``. The `as<>` action converts its argument to the requested type using Boost.Lexical_cast. The effect of the whole action is to insert a new word/integer pair into the map.

| Note
There is an important difference between the function `boost::ref()` in `<boost/ref.hpp>` and `boost::xpressive::ref()` in `<boost/xpressive/regex_actions.hpp>`. The first returns a plain `reference_wrapper<>` which behaves in many respects like an ordinary reference. By contrast, `boost::xpressive::ref()` returns a lazy reference that you can use in expressions that are executed lazily. That is why we can say `ref(result)[s1]`, even though `result` doesn't have an `operator[]` that would accept `s1`.

In addition to the sub-match placeholders `s1`, `s2`, etc., you can also use the placeholder `_` within an action to refer back to the string matched by the sub-expression to which the action is attached. For instance, you can use the following regex to match a bunch of digits, interpret them as an integer and assign the result to a local variable:

    int i = 0;
    // Here, _ refers back to all the
    // characters matched by (+_d)
    sregex rex = (+_d)[ ref(i) = as<int>(_) ];

#### Lazy Action Execution

What does it mean, exactly, to attach an action to part of a regular expression and perform a match? When does the action execute? If the action is part of a repeated sub-expression, does the action execute once or many times? And if the sub-expression initially matches, but ultimately fails because the rest of the regular expression fails to match, is the action executed at all?

The answer is that by default, actions are executed lazily. When a sub-expression matches a string, its action is placed on a queue, along with the current values of any sub-matches to which the action refers. If the match algorithm must backtrack, actions are popped off the queue as necessary. Only after the entire regex has matched successfully are the actions actually exeucted. They are executed all at once, in the order in which they were added to the queue, as the last step before ``regex_match()`` returns.

For example, consider the following regex that increments a counter whenever it finds a digit.

    int i = 0;
    std::string str("1!2!3?");
    // count the exciting digits, but not the
    // questionable ones.
    sregex rex = +( _d [ ++ref(i) ] >> '!' );
    regex_search(str, rex);
    assert( i == 2 );

The action `++ref(i)` is queued three times: once for each found digit. But it is only executed twice: once for each digit that precedes a `'!'` character. When the `'?'` character is encountered, the match algorithm backtracks, removing the final action from the queue.

#### Immediate Action Execution

When you want semantic actions to execute immediately, you can wrap the sub-expression containing the action in a ``keep()``. `keep()` turns off back-tracking for its sub-expression, but it also causes any actions queued by the sub-expression to execute at the end of the `keep()`. It is as if the sub-expression in the `keep()` were compiled into an independent regex object, and matching the `keep()` is like a separate invocation of `regex_search()`. It matches characters and executes actions but never backtracks or unwinds. For example, imagine the above example had been written as follows:

    int i = 0;
    std::string str("1!2!3?");
    // count all the digits.
    sregex rex = +( keep( _d [ ++ref(i) ] ) >> '!' );
    regex_search(str, rex);
    assert( i == 3 );

We have wrapped the sub-expression `_d [ ++ref(i) ]` in `keep()`. Now, whenever this regex matches a digit, the action will be queued and then immediately executed before we try to match a `'!'` character. In this case, the action executes three times.

| Note
Like `keep()`, actions within ``before()`` and ``after()`` are also executed early when their sub-expressions have matched.

#### Lazy Functions

So far, we've seen how to write semantic actions consisting of variables and operators. But what if you want to be able to call a function from a semantic action? Xpressive provides a mechanism to do this.

The first step is to define a function object type. Here, for instance, is a function object type that calls `push()` on its argument:

    struct push_impl
    {
        // Result type, needed for tr1::result_of
        typedef void result_type;

        template<typename Sequence, typename Value>
        void operator()(Sequence &seq, Value const &val) const
        {
            seq.push(val);
        }
    };

The next step is to use xpressive's `function<>` template to define a function object named `push`:

    // Global "push" function object.
    function<push_impl>::type const push = {{}};

The initialization looks a bit odd, but this is because `push` is being statically initialized. That means it doesn't need to be constructed at runtime. We can use `push` in semantic actions as follows:

    std::stack<int> ints;
    // Match digits, cast them to an int
    // and push it on the stack.
    sregex rex = (+_d)[push(ref(ints), as<int>(_))];

You'll notice that doing it this way causes member function invocations to look like ordinary function invocations. You can choose to write your semantic action in a different way that makes it look a bit more like a member function call:

    sregex rex = (+_d)[ref(ints)->*push(as<int>(_))];

Xpressive recognizes the use of the `->*` and treats this expression exactly the same as the one above.

When your function object must return a type that depends on its arguments, you can use a `result<>` member template instead of the `result_type` typedef. Here, for example, is a `first` function object that returns the `first` member of a `std::pair<>` or ``sub_match<>``:

    // Function object that returns the
    // first element of a pair.
    struct first_impl
    {
        template<typename Sig> struct result {};

        template<typename This, typename Pair>
        struct result<This(Pair)>
        {
            typedef typename remove_reference<Pair>
                ::type::first_type type;
        };

        template<typename Pair>
        typename Pair::first_type
        operator()(Pair const &p) const
        {
            return p.first;
        }
    };

    // OK, use as first(s1) to get the begin iterator
    // of the sub-match referred to by s1.
    function<first_impl>::type const first = {{}};

#### Referring to Local Variables

As we've seen in the examples above, we can refer to local variables within an actions using `xpressive::ref()`. Any such variables are held by reference by the regular expression, and care should be taken to avoid letting those references dangle. For instance, in the following code, the reference to `i` is left to dangle when `bad_voodoo()` returns:

    sregex bad_voodoo()
    {
        int i = 0;
        sregex rex = +( _d [ ++ref(i) ] >> '!' );
        // ERROR! rex refers by reference to a local
        // variable, which will dangle after bad_voodoo()
        // returns.
        return rex;
    }

When writing semantic actions, it is your responsibility to make sure that all the references do not dangle. One way to do that would be to make the variables shared pointers that are held by the regex by value.

    sregex good_voodoo(boost::shared_ptr<int> pi)
    {
        // Use val() to hold the shared_ptr by value:
        sregex rex = +( _d [ ++*val(pi) ] >> '!' );
        // OK, rex holds a reference count to the integer.
        return rex;
    }

In the above code, we use `xpressive::val()` to hold the shared pointer by value. That's not normally necessary because local variables appearing in actions are held by value by default, but in this case, it is necessary. Had we written the action as `++*pi`, it would have executed immediately. That's because `++*pi` is not an expression template, but `++*val(pi)` is.

It can be tedious to wrap all your variables in `ref()` and `val()` in your semantic actions. Xpressive provides the `reference<>` and `value<>` templates to make things easier. The following table shows the equivalencies:

Table 39.12. reference<> and value<>

This ... | ... is equivalent to this ...
--- | ---
    int i = 0;

    sregex rex = +( _d [ ++ref(i) ] >> '!' );
    int i = 0;
    reference<int> ri(i);
    sregex rex = +( _d [ ++ri ] >> '!' );
    boost::shared_ptr<int> pi(new int(0));

    sregex rex = +( _d [ ++*val(pi) ] >> '!' );
    boost::shared_ptr<int> pi(new int(0));
    value<boost::shared_ptr<int> > vpi(pi);
    sregex rex = +( _d [ ++*vpi ] >> '!' );

As you can see, when using `reference<>`, you need to first declare a local variable and then declare a `reference<>` to it. These two steps can be combined into one using `local<>`.

Table 39.13. local<> vs. reference<>

This ... | ... is equivalent to this ...
--- | ---
    local<int> i(0);

    sregex rex = +( _d [ ++i ] >> '!' );
    int i = 0;
    reference<int> ri(i);
    sregex rex = +( _d [ ++ri ] >> '!' );

We can use `local<>` to rewrite the above example as follows:

    local<int> i(0);
    std::string str("1!2!3?");
    // count the exciting digits, but not the
    // questionable ones.
    sregex rex = +( _d [ ++i ] >> '!' );
    regex_search(str, rex);
    assert( i.get() == 2 );

Notice that we use `local<>::get()` to access the value of the local variable. Also, beware that `local<>` can be used to create a dangling reference, just as `reference<>` can.

#### Referring to Non-Local Variables

In the beginning of this section, we used a regex with a semantic action to parse a string of word/integer pairs and stuff them into a `std::map<>`. That required that the map and the regex be defined together and used before either could go out of scope. What if we wanted to define the regex once and use it to fill lots of different maps? We would rather pass the map into the ``regex_match()`` algorithm rather than embed a reference to it directly in the regex object. What we can do instead is define a placeholder and use that in the semantic action instead of the map itself. Later, when we call one of the regex algorithms, we can bind the reference to an actual map object. The following code shows how.

    // Define a placeholder for a map object:
    placeholder<std::map<std::string, int> > _map;

    // Match a word and an integer, separated by =>,
    // and then stuff the result into a std::map<>
    sregex pair = ( (s1= +_w) >> "=>" >> (s2= +_d) )
        [ _map[s1] = as<int>(s2) ];

    // Match one or more word/integer pairs, separated
    // by whitespace.
    sregex rx = pair >> *(+_s >> pair);

    // The string to parse
    std::string str("aaa=>1 bbb=>23 ccc=>456");

    // Here is the actual map to fill in:
    std::map<std::string, int> result;

    // Bind the _map placeholder to the actual map
    smatch what;
    what.let( _map = result );

    // Execute the match and fill in result map
    if(regex_match(str, what, rx))
    {
        std::cout << result["aaa"] << '\n';
        std::cout << result["bbb"] << '\n';
        std::cout << result["ccc"] << '\n';
    }

This program displays:

    1
    23
    456

We use `placeholder<>` here to define `_map`, which stands in for a `std::map<>` variable. We can use the placeholder in the semantic action as if it were a map. Then, we define a ``match_results<>`` struct and bind an actual map to the placeholder with "`what.let( _map = result );`". The ``regex_match()`` call behaves as if the placeholder in the semantic action had been replaced with a reference to `result`.

| Note
Placeholders in semantic actions are not actually replaced at runtime with references to variables. The regex object is never mutated in any way during any of the regex algorithms, so they are safe to use in multiple threads.

The syntax for late-bound action arguments is a little different if you are using ``regex_iterator<>`` or ``regex_token_iterator<>``. The regex iterators accept an extra constructor parameter for specifying the argument bindings. There is a `let()` function that you can use to bind variables to their placeholders. The following code demonstrates how.

    // Define a placeholder for a map object:
    placeholder<std::map<std::string, int> > _map;

    // Match a word and an integer, separated by =>,
    // and then stuff the result into a std::map<>
    sregex pair = ( (s1= +_w) >> "=>" >> (s2= +_d) )
        [ _map[s1] = as<int>(s2) ];

    // The string to parse
    std::string str("aaa=>1 bbb=>23 ccc=>456");

    // Here is the actual map to fill in:
    std::map<std::string, int> result;

    // Create a regex_iterator to find all the matches
    sregex_iterator it(str.begin(), str.end(), pair, let(_map=result));
    sregex_iterator end;

    // step through all the matches, and fill in
    // the result map
    while(it != end)
        ++it;

    std::cout << result["aaa"] << '\n';
    std::cout << result["bbb"] << '\n';
    std::cout << result["ccc"] << '\n';

This program displays:

    1
    23
    456

### User-Defined Assertions

You are probably already familiar with regular expression assertions. In Perl, some examples are the `^` and `$` assertions, which you can use to match the beginning and end of a string, respectively. Xpressive lets you define your own assertions. A custom assertion is a contition which must be true at a point in the match in order for the match to succeed. You can check a custom assertion with xpressive's ``check()`` function.

There are a couple of ways to define a custom assertion. The simplest is to use a function object. Let's say that you want to ensure that a sub-expression matches a sub-string that is either 3 or 6 characters long. The following struct defines such a predicate:

    // A predicate that is true IFF a sub-match is
    // either 3 or 6 characters long.
    struct three_or_six
    {
        bool operator()(ssub_match const &sub) const
        {
            return sub.length() == 3 || sub.length() == 6;
        }
    };

You can use this predicate within a regular expression as follows:

    // match words of 3 characters or 6 characters.
    sregex rx = (bow >> +_w >> eow)[ check(three_or_six()) ] ;

The above regular expression will find whole words that are either 3 or 6 characters long. The `three_or_six` predicate accepts a ``sub_match<>`` that refers back to the part of the string matched by the sub-expression to which the custom assertion is attached.

| Note
The custom assertion participates in determining whether the match succeeds or fails. Unlike actions, which execute lazily, custom assertions execute immediately while the regex engine is searching for a match.

Custom assertions can also be defined inline using the same syntax as for semantic actions. Below is the same custom assertion written inline:

    // match words of 3 characters or 6 characters.
    sregex rx = (bow >> +_w >> eow)[ check(length(_)==3 || length(_)==6) ] ;

In the above, `length()` is a lazy function that calls the `length()` member function of its argument, and `_` is a placeholder that receives the `sub_match`.

Once you get the hang of writing custom assertions inline, they can be very powerful. For example, you can write a regular expression that only matches valid dates (for some suitably liberal definition of the term “valid”).

    int const days_per_month[] =
        {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 31, 31};

    mark_tag month(1), day(2);
    // find a valid date of the form month/day/year.
    sregex date =
        (
            // Month must be between 1 and 12 inclusive
            (month= _d >> !_d)     [ check(as<int>(_) >= 1
                                        && as<int>(_) <= 12) ]
        >>  '/'
            // Day must be between 1 and 31 inclusive
        >>  (day=   _d >> !_d)     [ check(as<int>(_) >= 1
                                        && as<int>(_) <= 31) ]
        >>  '/'
            // Only consider years between 1970 and 2038
        >>  (_d >> _d >> _d >> _d) [ check(as<int>(_) >= 1970
                                        && as<int>(_) <= 2038) ]
        )
        // Ensure the month actually has that many days!
        [ check( ref(days_per_month)[as<int>(month)-1] >= as<int>(day) ) ]
    ;

    smatch what;
    std::string str("99/99/9999 2/30/2006 2/28/2006");

    if(regex_search(str, what, date))
    {
        std::cout << what[0] << std::endl;
    }

The above program prints out the following:

    2/28/2006

Notice how the inline custom assertions are used to range-check the values for the month, day and year. The regular expression doesn't match `"99/99/9999"` or `"2/30/2006"` because they are not valid dates. (There is no 99th month, and February doesn't have 30 days.)

### Symbol Tables and Attributes

### Overview

Symbol tables can be built into xpressive regular expressions with just a `std::map<>`. The map keys are the strings to be matched and the map values are the data to be returned to your semantic action. Xpressive attributes, named `a1`, `a2`, through `a9`, hold the value corresponding to a matching key so that it can be used in a semantic action. A default value can be specified for an attribute if a symbol is not found.

### Symbol Tables

An xpressive symbol table is just a `std::map<>`, where the key is a string type and the value can be anything. For example, the following regular expression matches a key from map1 and assigns the corresponding value to the attribute `a1`. Then, in the semantic action, it assigns the value stored in attribute `a1` to an integer result.

    int result;
    std::map<std::string, int> map1;
    // ... (fill the map)
    sregex rx = ( a1 = map1 ) [ ref(result) = a1 ];

Consider the following example code, which translates number names into integers. It is described below.

    #include <string>
    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>
    #include <boost/xpressive/regex_actions.hpp>
    using namespace boost::xpressive;

    int main()
    {
        std::map<std::string, int> number_map;
        number_map["one"] = 1;
        number_map["two"] = 2;
        number_map["three"] = 3;
        // Match a string from number_map
        // and store the integer value in 'result'
        // if not found, store -1 in 'result'
        int result = 0;
        cregex rx = ((a1 = number_map ) | *_)
            [ ref(result) = (a1 | -1)];

        regex_match("three", rx);
        std::cout << result << '\n';
        regex_match("two", rx);
        std::cout << result << '\n';
        regex_match("stuff", rx);
        std::cout << result << '\n';
        return 0;
    }

This program prints the following:

    3
    2
    -1

First the program builds a number map, with number names as string keys and the corresponding integers as values. Then it constructs a static regular expression using an attribute `a1` to represent the result of the symbol table lookup. In the semantic action, the attribute is assigned to an integer variable `result`. If the symbol was not found, a default value of `-1` is assigned to `result`. A wildcard, `*_`, makes sure the regex matches even if the symbol is not found.

A more complete version of this example can be found in `libs/xpressive/example/numbers.cpp`[10]. It translates number names up to "nine hundred ninety nine million nine hundred ninety nine thousand nine hundred ninety nine" along with some special number names like "dozen".

Symbol table matches are case sensitive by default, but they can be made case-insensitive by enclosing the expression in `icase()`.

### Attributes

Up to nine attributes can be used in a regular expression. They are named `a1`, `a2`, ..., `a9` in the `boost::xpressive` namespace. The attribute type is the same as the second component of the map that is assigned to it. A default value for an attribute can be specified in a semantic action with the syntax `(a1 | `default-value`)`.

Attributes are properly scoped, so you can do crazy things like: `( (a1=sym1) >> (a1=sym2)[ref(x)=a1] )[ref(y)=a1]`. The inner semantic action sees the inner `a1`, and the outer semantic action sees the outer one. They can even have different types.

| Note
Xpressive builds a hidden ternary search trie from the map so it can search quickly. If BOOST_DISABLE_THREADS is defined, the hidden ternary search trie "self adjusts", so after each search it restructures itself to improve the efficiency of future searches based on the frequency of previous searches.

### Localization and Regex Traits

### Overview

Matching a regular expression against a string often requires locale-dependent information. For example, how are case-insensitive comparisons performed? The locale-sensitive behavior is captured in a traits class. xpressive provides three traits class templates: `cpp_regex_traits<>`, `c_regex_traits<>` and `null_regex_traits<>`. The first wraps a `std::locale`, the second wraps the global C locale, and the third is a stub traits type for use when searching non-character data. All traits templates conform to the Regex Traits Concept.

### Setting the Default Regex Trait

By default, xpressive uses `cpp_regex_traits<>` for all patterns. This causes all regex objects to use the global `std::locale`. If you compile with `BOOST_XPRESSIVE_USE_C_TRAITS` defined, then xpressive will use `c_regex_traits<>` by default.

### Using Custom Traits with Dynamic Regexes

To create a dynamic regex that uses a custom traits object, you must use ``regex_compiler<>``. The basic steps are shown in the following example:

    // Declare a regex_compiler that uses the global C locale
    regex_compiler<char const *, c_regex_traits<char> > crxcomp;
    cregex crx = crxcomp.compile( "\\w+" );

    // Declare a regex_compiler that uses a custom std::locale
    std::locale loc = /* ... create a locale here ... */;
    regex_compiler<char const *, cpp_regex_traits<char> > cpprxcomp(loc);
    cregex cpprx = cpprxcomp.compile( "\\w+" );

The `regex_compiler` objects act as regex factories. Once they have been imbued with a locale, every regex object they create will use that locale.

### Using Custom Traits with Static Regexes

If you want a particular static regex to use a different set of traits, you can use the special `imbue()` pattern modifier. For instance:

    // Define a regex that uses the global C locale
    c_regex_traits<char> ctraits;
    sregex crx = imbue(ctraits)( +_w );

    // Define a regex that uses a customized std::locale
    std::locale loc = /* ... create a locale here ... */;
    cpp_regex_traits<char> cpptraits(loc);
    sregex cpprx1 = imbue(cpptraits)( +_w );

    // A shorthand for above
    sregex cpprx2 = imbue(loc)( +_w );

The `imbue()` pattern modifier must wrap the entire pattern. It is an error to `imbue` only part of a static regex. For example:

    // ERROR! Cannot imbue() only part of a regex
    sregex error = _w >> imbue(loc)( _w );

### Searching Non-Character Data With `null_regex_traits`

With xpressive static regexes, you are not limitted to searching for patterns in character sequences. You can search for patterns in raw bytes, integers, or anything that conforms to the Char Concept. The `null_regex_traits<>` makes it simple. It is a stub implementation of the Regex Traits Concept. It recognizes no character classes and does no case-sensitive mappings.

For example, with `null_regex_traits<>`, you can write a static regex to find a pattern in a sequence of integers as follows:

    // some integral data to search
    int const data[] = {0, 1, 2, 3, 4, 5, 6};

    // create a null_regex_traits<> object for searching integers ...
    null_regex_traits<int> nul;

    // imbue a regex object with the null_regex_traits ...
    basic_regex<int const *> rex = imbue(nul)(1 >> +((set= 2,3) | 4) >> 5);
    match_results<int const *> what;

    // search for the pattern in the array of integers ...
    regex_search(data, data + 7, what, rex);

    assert(what[0].matched);
    assert(*what[0].first == 1);
    assert(*what[0].second == 6);

### Tips 'N Tricks

Squeeze the most performance out of xpressive with these tips and tricks.

### Compile Patterns Once And Reuse Them

Compiling a regex (dynamic or static) is far more expensive than executing a match or search. If you have the option, prefer to compile a pattern into a ``basic_regex<>`` object once and reuse it rather than recreating it over and over.

Since ``basic_regex<>`` objects are not mutated by any of the regex algorithms, they are completely thread-safe once their initialization (and that of any grammars of which they are members) completes. The easiest way to reuse your patterns is to simply make your ``basic_regex<>`` objects "static const".

### Reuse match_results<> Objects

The ``match_results<>`` object caches dynamically allocated memory. For this reason, it is far better to reuse the same ``match_results<>`` object if you have to do many regex searches.

Caveat: ``match_results<>`` objects are not thread-safe, so don't go wild reusing them across threads.

### Prefer Algorithms That Take A match_results<> Object

This is a corollary to the previous tip. If you are doing multiple searches, you should prefer the regex algorithms that accept a ``match_results<>`` object over the ones that don't, and you should reuse the same ``match_results<>`` object each time. If you don't provide a ``match_results<>`` object, a temporary one will be created for you and discarded when the algorithm returns. Any memory cached in the object will be deallocated and will have to be reallocated the next time.

### Prefer Algorithms That Accept Iterator Ranges Over Null-Terminated Strings

xpressive provides overloads of the ``regex_match()`` and ``regex_search()`` algorithms that operate on C-style null-terminated strings. You should prefer the overloads that take iterator ranges. When you pass a null-terminated string to a regex algorithm, the end iterator is calculated immediately by calling `strlen`. If you already know the length of the string, you can avoid this overhead by calling the regex algorithms with a `[begin, end)` pair.

### Use Static Regexes

On average, static regexes execute about 10 to 15% faster than their dynamic counterparts. It's worth familiarizing yourself with the static regex dialect.

### Understand `syntax_option_type::optimize`

The `optimize` flag tells the regex compiler to spend some extra time analyzing the pattern. It can cause some patterns to execute faster, but it increases the time to compile the pattern, and often increases the amount of memory consumed by the pattern. If you plan to reuse your pattern, `optimize` is usually a win. If you will only use the pattern once, don't use `optimize`.

## Common Pitfalls

Keep the following tips in mind to avoid stepping in potholes with xpressive.

### Create Grammars On A Single Thread

With static regexes, you can create grammars by nesting regexes inside one another. When compiling the outer regex, both the outer and inner regex objects, and all the regex objects to which they refer either directly or indirectly, are modified. For this reason, it's dangerous for global regex objects to participate in grammars. It's best to build regex grammars from a single thread. Once built, the resulting regex grammar can be executed from multiple threads without problems.

### Beware Nested Quantifiers

This is a pitfall common to many regular expression engines. Some patterns can cause exponentially bad performance. Often these patterns involve one quantified term nested withing another quantifier, such as `"(a*)*"`, although in many cases, the problem is harder to spot. Beware of patterns that have nested quantifiers.

### Concepts

### CharT requirements

If type `BidiIterT` is used as a template argument to ``basic_regex<>``, then `CharT` is `iterator_traits<BidiIterT>::value_type`. Type `CharT` must have a trivial default constructor, copy constructor, assignment operator, and destructor. In addition the following requirements must be met for objects; `c` of type `CharT`, `c1` and `c2` of type `CharT const`, and `i` of type `int`:

Table 39.14. CharT Requirements

Expression | Return type | Assertion / Note / Pre- / Post-condition
--- | --- | ---
`CharT c` | `CharT` | Default constructor (must be trivial).
`CharT c(c1)` | `CharT` | Copy constructor (must be trivial).
`c1 = c2` | `CharT` | Assignment operator (must be trivial).
`c1 == c2` | `bool` | `true` if `c1` has the same value as `c2`.
`c1 != c2` | `bool` | `true` if `c1` and `c2` are not equal.
`c1 < c2` | `bool` | `true` if the value of `c1` is less than `c2`.
`c1 > c2` | `bool` | `true` if the value of `c1` is greater than `c2`.
`c1 <= c2` | `bool` | `true` if `c1` is less than or equal to `c2`.
`c1 >= c2` | `bool` | `true` if `c1` is greater than or equal to `c2`.
`intmax_t i = c1` | `int` | `CharT` must be convertible to an integral type.
`CharT c(i);` | `CharT` | `CharT` must be constructable from an integral type.

### Traits Requirements

In the following table `X` denotes a traits class defining types and functions for the character container type `CharT`; `u` is an object of type `X`; `v` is an object of type `const X`; `p` is a value of type `const CharT*`; `I1` and `I2` are `Input Iterators`; `c` is a value of type `const CharT`; `s` is an object of type `X::string_type`; `cs` is an object of type `const X::string_type`; `b` is a value of type `bool`; `i` is a value of type `int`; `F1` and `F2` are values of type `const CharT*`; `loc` is an object of type `X::locale_type`; and `ch` is an object of `const char`.

Table 39.15. Traits Requirements

Expression | Return type | Assertion / Note Pre / Post condition
--- | --- | ---
`X::char_type` | `CharT` | The character container type used in the implementation of class template ``basic_regex<>``.
`X::string_type` | `std::basic_string<CharT>` or `std::vector<CharT>` |
`X::locale_type` | Implementation defined | A copy constructible type that represents the locale used by the traits class.
`X::char_class_type` | Implementation defined | A bitmask type representing a particular character classification. Multiple values of this type can be bitwise-or'ed together to obtain a new valid value.
`X::hash(c)` | `unsigned char` | Yields a value between `0` and `UCHAR_MAX` inclusive.
`v.widen(ch)` | `CharT` | Widens the specified `char` and returns the resulting `CharT`.
`v.in_range(r1, r2, c)` | `bool` | For any characters `r1` and `r2`, returns `true` if `r1 <= c && c <= r2`. Requires that `r1 <= r2`.
`v.in_range_nocase(r1, r2, c)` | `bool` | For characters `r1` and `r2`, returns `true` if there is some character `d` for which `v.translate_nocase(d) == v.translate_nocase(c)` and `r1 <= d && d <= r2`. Requires that `r1 <= r2`.
`v.translate(c)` | `X::char_type` | Returns a character such that for any character `d` that is to be considered equivalent to `c` then `v.translate(c) == v.translate(d)`.
`v.translate_nocase(c)` | `X::char_type` | For all characters `C` that are to be considered equivalent to `c` when comparisons are to be performed without regard to case, then `v.translate_nocase(c) == v.translate_nocase(C)`.
`v.transform(F1, F2)` | `X::string_type` | Returns a sort key for the character sequence designated by the iterator range `[F1, F2)` such that if the character sequence `[G1, G2)` sorts before the character sequence `[H1, H2)` then `v.transform(G1, G2) < v.transform(H1, H2)`.
`v.transform_primary(F1, F2)` | `X::string_type` | Returns a sort key for the character sequence designated by the iterator range `[F1, F2)` such that if the character sequence `[G1, G2)` sorts before the character sequence `[H1, H2)` when character case is not considered then `v.transform_primary(G1, G2) < v.transform_primary(H1, H2)`.
`v.lookup_classname(F1, F2)` | `X::char_class_type` | Converts the character sequence designated by the iterator range `[F1,F2)` into a bitmask type that can subsequently be passed to `isctype`. Values returned from `lookup_classname` can be safely bitwise or'ed together. Returns `0` if the character sequence is not the name of a character class recognized by `X`. The value returned shall be independent of the case of the characters in the sequence.
`v.lookup_collatename(F1, F2)` | `X::string_type` | Returns a sequence of characters that represents the collating element consisting of the character sequence designated by the iterator range `[F1, F2)`. Returns an empty string if the character sequence is not a valid collating element.
`v.isctype(c, v.lookup_classname(F1, F2))` | `bool` | Returns `true` if character `c` is a member of the character class designated by the iterator range `[F1, F2)`, `false` otherwise.
`v.value(c, i)` | `int` | Returns the value represented by the digit `c` in base `i` if the character `c` is a valid digit in base `i`; otherwise returns `-1`. [Note: the value of `i` will only be `8`, `10`, or `16`. -end note]
`u.imbue(loc)` | `X::locale_type` | Imbues `u` with the locale `loc`, returns the previous locale used by `u`.
`v.getloc()` | `X::locale_type` | Returns the current locale used by `v`.

### Acknowledgements

This section is adapted from the equivalent page in the Boost.Regex documentation and from the proposal to add regular expressions to the Standard Library.

### Examples

Below you can find six complete sample programs.

##### See if a whole string matches a regex

This is the example from the Introduction. It is reproduced here for your convenience.

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        std::string hello( "hello world!" );

        sregex rex = sregex::compile( "(\\w+) (\\w+)!" );
        smatch what;

        if( regex_match( hello, what, rex ) )
        {
            std::cout << what[0] << '\n'; // whole match
            std::cout << what[1] << '\n'; // first capture
            std::cout << what[2] << '\n'; // second capture
        }

        return 0;
    }

This program outputs the following:

    hello world!
    hello
    world

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##### See if a string contains a sub-string that matches a regex

Notice in this example how we use custom `mark_tag`s to make the pattern more readable. We can use the `mark_tag`s later to index into the ``match_results<>``.

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        char const *str = "I was born on 5/30/1973 at 7am.";

        // define some custom mark_tags with names more meaningful than s1, s2, etc.
        mark_tag day(1), month(2), year(3), delim(4);

        // this regex finds a date
        cregex date = (month= repeat<1,2>(_d))           // find the month ...
                   >> (delim= (set= '/','-'))            // followed by a delimiter ...
                   >> (day=   repeat<1,2>(_d)) >> delim  // and a day followed by the same delimiter ...
                   >> (year=  repeat<1,2>(_d >> _d));    // and the year.

        cmatch what;

        if( regex_search( str, what, date ) )
        {
            std::cout << what[0]     << '\n'; // whole match
            std::cout << what[day]   << '\n'; // the day
            std::cout << what[month] << '\n'; // the month
            std::cout << what[year]  << '\n'; // the year
            std::cout << what[delim] << '\n'; // the delimiter
        }

        return 0;
    }

This program outputs the following:

    5/30/1973
    30
    5
    1973
    /

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##### Replace all sub-strings that match a regex

The following program finds dates in a string and marks them up with pseudo-HTML.

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        std::string str( "I was born on 5/30/1973 at 7am." );

        // essentially the same regex as in the previous example, but using a dynamic regex
        sregex date = sregex::compile( "(\\d{1,2})([/-])(\\d{1,2})\\2((?:\\d{2}){1,2})" );

        // As in Perl, $& is a reference to the sub-string that matched the regex
        std::string format( "<date>$&</date>" );

        str = regex_replace( str, date, format );
        std::cout << str << '\n';

        return 0;
    }

This program outputs the following:

    I was born on <date>5/30/1973</date> at 7am.

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##### Find all the sub-strings that match a regex and step through them one at a time

The following program finds the words in a wide-character string. It uses `wsregex_iterator`. Notice that dereferencing a `wsregex_iterator` yields a `wsmatch` object.

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        std::wstring str( L"This is his face." );

        // find a whole word
        wsregex token = +alnum;

        wsregex_iterator cur( str.begin(), str.end(), token );
        wsregex_iterator end;

        for( ; cur != end; ++cur )
        {
            wsmatch const &what = *cur;
            std::wcout << what[0] << L'\n';
        }

        return 0;
    }

This program outputs the following:

    This
    is
    his
    face

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##### Split a string into tokens that each match a regex

The following program finds race times in a string and displays first the minutes and then the seconds. It uses ``regex_token_iterator<>``.

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        std::string str( "Eric: 4:40, Karl: 3:35, Francesca: 2:32" );

        // find a race time
        sregex time = sregex::compile( "(\\d):(\\d\\d)" );

        // for each match, the token iterator should first take the value of
        // the first marked sub-expression followed by the value of the second
        // marked sub-expression
        int const subs[] = { 1, 2 };

        sregex_token_iterator cur( str.begin(), str.end(), time, subs );
        sregex_token_iterator end;

        for( ; cur != end; ++cur )
        {
            std::cout << *cur << '\n';
        }

        return 0;
    }

This program outputs the following:

    4
    40
    3
    35
    2
    32

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##### Split a string using a regex as a delimiter

The following program takes some text that has been marked up with html and strips out the mark-up. It uses a regex that matches an HTML tag and a ``regex_token_iterator<>`` that returns the parts of the string that do not match the regex.

    #include <iostream>
    #include <boost/xpressive/xpressive.hpp>

    using namespace boost::xpressive;

    int main()
    {
        std::string str( "Now <bold>is the time <i>for all good men</i> to come to the aid of their</bold> country." );

        // find a HTML tag
        sregex html = '<' >> optional('/') >> +_w >> '>';

        // the -1 below directs the token iterator to display the parts of
        // the string that did NOT match the regular expression.
        sregex_token_iterator cur( str.begin(), str.end(), html, -1 );
        sregex_token_iterator end;

        for( ; cur != end; ++cur )
        {
            std::cout << '{' << *cur << '}';
        }
        std::cout << '\n';

        return 0;
    }

This program outputs the following:

    {Now }{is the time }{for all good men}{ to come to the aid of their}{ country.}

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##### Display a tree of nested results

Here is a helper class to demonstrate how you might display a tree of nested results:

    // Displays nested results to std::cout with indenting
    struct output_nested_results
    {
        int tabs_;

        output_nested_results( int tabs = 0 )
            : tabs_( tabs )
        {
        }

        template< typename BidiIterT >
        void operator ()( match_results< BidiIterT > const &what ) const
        {
            // first, do some indenting
            typedef typename std::iterator_traits< BidiIterT >::value_type char_type;
            char_type space_ch = char_type(' ');
            std::fill_n( std::ostream_iterator<char_type>( std::cout ), tabs_ * 4, space_ch );

            // output the match
            std::cout << what[0] << '\n';

            // output any nested matches
            std::for_each(
                what.nested_results().begin(),
                what.nested_results().end(),
                output_nested_results( tabs_ + 1 ) );
        }
    };

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[9] See Expression Templates

[10] Many thanks to David Jenkins, who contributed this example.