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4      <title>Boost.Regex: Introduction</title>
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16                  <H1 align="center">Boost.Regex</H1>
17                  <H2 align="center">Introduction</H2>
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26      <p></p>
27      <P>Regular expressions are a form of pattern-matching that are often used in text
28         processing; many users will be familiar with the Unix utilities <I>grep</I>, <I>sed</I>
29         and <I>awk</I>, and the programming language <I>Perl</I>, each of which make
30         extensive use of regular expressions. Traditionally C++ users have been limited
31         to the POSIX C API's for manipulating regular expressions, and while regex++
32         does provide these API's, they do not represent the best way to use the
33         library. For example regex++ can cope with wide character strings, or search
34         and replace operations (in a manner analogous to either sed or Perl), something
35         that traditional C libraries can not do.</P>
36      <P>The class <A href="basic_regex.html">boost::basic_regex</A> is the key class in
37         this library; it represents a "machine readable" regular expression, and is
38         very closely modeled on std::basic_string, think of it as a string plus the
39         actual state-machine required by the regular expression algorithms. Like
40         std::basic_string there are two typedefs that are almost always the means by
41         which this class is referenced:</P>
42      <pre><B>namespace </B>boost{
43
44<B>template</B> &lt;<B>class</B> charT,
45<B>          class</B> traits = regex_traits&lt;charT&gt; &gt;
46<B>class</B> basic_regex;
47
48<B>typedef</B> basic_regex&lt;<B>char</B>&gt; regex;
49<B>typedef</B> basic_regex&lt;<B>wchar_t&gt;</B> wregex;
50
51}</pre>
52      <P>To see how this library can be used, imagine that we are writing a credit card
53         processing application. Credit card numbers generally come as a string of
54         16-digits, separated into groups of 4-digits, and separated by either a space
55         or a hyphen. Before storing a credit card number in a database (not necessarily
56         something your customers will appreciate!), we may want to verify that the
57         number is in the correct format. To match any digit we could use the regular
58         expression [0-9], however ranges of characters like this are actually locale
59         dependent. Instead we should use the POSIX standard form [[:digit:]], or the
60         regex++ and Perl shorthand for this \d (note that many older libraries tended
61         to be hard-coded to the C-locale, consequently this was not an issue for them).
62         That leaves us with the following regular expression to validate credit card
63         number formats:</P>
64      <PRE>(\d{4}[- ]){3}\d{4}</PRE>
65      <P>Here the parenthesis act to group (and mark for future reference)
66         sub-expressions, and the {4} means "repeat exactly 4 times". This is an example
67         of the extended regular expression syntax used by Perl, awk and egrep. Regex++
68         also supports the older "basic" syntax used by sed and grep, but this is
69         generally less useful, unless you already have some basic regular expressions
70         that you need to reuse.</P>
71      <P>Now let's take that expression and place it in some C++ code to validate the
72         format of a credit card number:</P>
73      <PRE><B>bool</B> validate_card_format(<B>const</B> std::string s)
74{
75   <B>static</B> <B>const</B> <A href="basic_regex.html">boost::regex</A> e("(\\d{4}[- ]){3}\\d{4}");
76   <B>return</B> <A href="regex_match.html">regex_match</A>(s, e);
77}</PRE>
78      <P>Note how we had to add some extra escapes to the expression: remember that the
79         escape is seen once by the C++ compiler, before it gets to be seen by the
80         regular expression engine, consequently escapes in regular expressions have to
81         be doubled up when embedding them in C/C++ code. Also note that all the
82         examples assume that your compiler supports Koenig lookup, if yours doesn't
83         (for example VC6), then you will have to add some boost:: prefixes to some of
84         the function calls in the examples.</P>
85      <P>Those of you who are familiar with credit card processing, will have realized
86         that while the format used above is suitable for human readable card numbers,
87         it does not represent the format required by online credit card systems; these
88         require the number as a string of 16 (or possibly 15) digits, without any
89         intervening spaces. What we need is a means to convert easily between the two
90         formats, and this is where search and replace comes in. Those who are familiar
91         with the utilities <I>sed</I> and <I>Perl</I> will already be ahead here; we
92         need two strings - one a regular expression - the other a "<A href="format_syntax.html">format
93            string</A>" that provides a description of the text to replace the match
94         with. In regex++ this search and replace operation is performed with the
95         algorithm<A href="regex_replace.html"> regex_replace</A>, for our credit card
96         example we can write two algorithms like this to provide the format
97         conversions:</P>
98      <PRE><I>// match any format with the regular expression:
99</I><B>const</B> boost::regex e("\\A(\\d{3,4})[- ]?(\\d{4})[- ]?(\\d{4})[- ]?(\\d{4})\\z");
100<B>const</B> std::string machine_format("\\1\\2\\3\\4");
101<B>const</B> std::string human_format("\\1-\\2-\\3-\\4");
102
103std::string machine_readable_card_number(<B>const</B> std::string s)
104{
105   <B>return</B> <A href="regex_replace.html">regex_replace</A>(s, e, machine_format, boost::match_default | boost::format_sed);
106}
107
108std::string human_readable_card_number(<B>const</B> std::string s)
109{
110   <B>return</B> <A href="regex_replace.html">regex_replace</A>(s, e, human_format, boost::match_default | boost::format_sed);
111}</PRE>
112      <P>Here we've used marked sub-expressions in the regular expression to split out
113         the four parts of the card number as separate fields, the format string then
114         uses the sed-like syntax to replace the matched text with the reformatted
115         version.</P>
116      <P>In the examples above, we haven't directly manipulated the results of a regular
117         expression match, however in general the result of a match contains a number of
118         sub-expression matches in addition to the overall match. When the library needs
119         to report a regular expression match it does so using an instance of the class <A href="match_results.html">
120            match_results</A>, as before there are typedefs of this class for the most
121         common cases:
122      </P>
123      <PRE><B>namespace </B>boost{
124<B>typedef</B> match_results&lt;<B>const</B> <B>char</B>*&gt; cmatch;
125<B>typedef</B> match_results&lt;<B>const</B> <B>wchar_t</B>*&gt; wcmatch;
126<STRONG>typedef</STRONG> match_results&lt;std::string::const_iterator&gt; smatch;
127<STRONG>typedef</STRONG> match_results&lt;std::wstring::const_iterator&gt; wsmatch;
128}</PRE>
129      <P>The algorithms <A href="regex_search.html">regex_search</A> and&nbsp;<A href="regex_match.html">regex_match</A>
130         make use of match_results to report what matched; the difference between these
131         algorithms is that <A href="regex_match.html">regex_match</A> will only find
132         matches that consume <EM>all</EM> of the input text, where as <A href="regex_search.html">
133            regex_search</A> will <EM>search</EM> for a match anywhere within the text
134         being matched.</P>
135      <P>Note that these algorithms are not restricted to searching regular C-strings,
136         any bidirectional iterator type can be searched, allowing for the possibility
137         of seamlessly searching almost any kind of data.
138      </P>
139      <P>For search and replace operations, in addition to the algorithm <A href="regex_replace.html">
140            regex_replace</A> that we have already seen, the <A href="match_results.html">match_results</A>
141         class has a format member that takes the result of a match and a format string,
142         and produces a new string by merging the two.</P>
143      <P>For iterating through all occurences of an expression within a text, there are
144         two iterator types: <A href="regex_iterator.html">regex_iterator</A> will
145         enumerate over the <A href="match_results.html">match_results</A> objects
146         found, while <A href="regex_token_iterator.html">regex_token_iterator</A> will
147         enumerate a series of strings (similar to perl style split operations).</P>
148      <P>For those that dislike templates, there is a high level wrapper class RegEx
149         that is an encapsulation of the lower level template code - it provides a
150         simplified interface for those that don't need the full power of the library,
151         and supports only narrow characters, and the "extended" regular expression
152         syntax. This class is now deprecated as it does not form part of the regular
153         expressions C++ standard library proposal.
154      </P>
155      <P>The <A href="posix_api.html">POSIX API</A> functions: regcomp, regexec, regfree
156         and regerror, are available in both narrow character and Unicode versions, and
157         are provided for those who need compatibility with these API's.
158      </P>
159      <P>Finally, note that the library now has run-time <A href="localisation.html">localization</A>
160         support, and recognizes the full POSIX regular expression syntax - including
161         advanced features like multi-character collating elements and equivalence
162         classes - as well as providing compatibility with other regular expression
163         libraries including GNU and BSD4 regex packages, and to a more limited extent
164         Perl 5.
165      </P>
166      <P>
167         <HR>
168      <P></P>
169      <p>Revised
170         <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> 
171         24 Oct 2003
172         <!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
173      <p><i>© Copyright John Maddock&nbsp;1998-
174            <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y" startspan --> 
175            2003<!--webbot bot="Timestamp" endspan i-checksum="39359" --></i></p>
176      <P><I>Use, modification and distribution are subject to the Boost Software License,
177            Version 1.0. (See accompanying file <A href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A>
178            or copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)</I></P>
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