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- [/
- / Copyright (c) 2008 Eric Niebler
- /
- / Distributed under the Boost Software License, Version 1.0. (See accompanying
- / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
- /]
- [section Static Regexes]
- [h2 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++.
- [h2 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 = '$' >> +_d >> '.' >> _d >> _d;
- Assignment works similarly.
- [h2 Character and String Literals]
- In static regexes, character and string literals match themselves. For
- instance, in the regex above, `'$'` 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
- [h2 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 );
- [h2 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] \n\n 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.]
- [h2 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 [link boost_xpressive.user_s_guide.localization_and_regex_traits
- Localization and Regex Traits] for more information about how to customize
- the behavior of your regexes.
- [h2 Static xpressive Syntax Cheat Sheet]
- The table below lists the familiar regex constructs and their equivalents in
- static xpressive.
- [def _s1_ [globalref boost::xpressive::s1 s1]]
- [def _bos_ [globalref boost::xpressive::bos bos]]
- [def _eos_ [globalref boost::xpressive::eos eos]]
- [def _b_ [globalref boost::xpressive::_b _b]]
- [def _n_ [globalref boost::xpressive::_n _n]]
- [def _ln_ [globalref boost::xpressive::_ln _ln]]
- [def _d_ [globalref boost::xpressive::_d _d]]
- [def _w_ [globalref boost::xpressive::_w _w]]
- [def _s_ [globalref boost::xpressive::_s _s]]
- [def _alnum_ [globalref boost::xpressive::alnum alnum]]
- [def _alpha_ [globalref boost::xpressive::alpha alpha]]
- [def _blank_ [globalref boost::xpressive::blank blank]]
- [def _cntrl_ [globalref boost::xpressive::cntrl cntrl]]
- [def _digit_ [globalref boost::xpressive::digit digit]]
- [def _graph_ [globalref boost::xpressive::graph graph]]
- [def _lower_ [globalref boost::xpressive::lower lower]]
- [def _print_ [globalref boost::xpressive::print print]]
- [def _punct_ [globalref boost::xpressive::punct punct]]
- [def _space_ [globalref boost::xpressive::space space]]
- [def _upper_ [globalref boost::xpressive::upper upper]]
- [def _xdigit_ [globalref boost::xpressive::xdigit xdigit]]
- [def _set_ [globalref boost::xpressive::set set]]
- [def _repeat_ [funcref boost::xpressive::repeat repeat]]
- [def _range_ [funcref boost::xpressive::range range]]
- [def _icase_ [funcref boost::xpressive::icase icase]]
- [def _before_ [funcref boost::xpressive::before before]]
- [def _after_ [funcref boost::xpressive::after after]]
- [def _keep_ [funcref boost::xpressive::keep keep]]
- [table Perl syntax vs. Static xpressive syntax
- [[Perl] [Static xpressive] [Meaning]]
- [[[^.]] [[globalref boost::xpressive::_ `_`]] [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]`);`\n ...\n `(`[^['name]]`= `[^['stuff]]`)`] [Create a named capture.]]
- [[[^(?P=['name])]] [`_mark_tag_ `[^['name]]`(`['n]`);`\n ...\n [^['name]]] [Refer back to a previously created named capture.]]
- ]
- \n
- [endsect]
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