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1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
2	<html>
3	<head>
4	<title>Boost.Regex: Perl Regular Expression Syntax</title>
5	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
6	<LINK href="../../../boost.css" type="text/css" rel="stylesheet"></head>
7	<body>
8	<P>
9	<TABLE id="Table1" cellSpacing="1" cellPadding="1" width="100%" border="0">
10	<TR>
11	<td vAlign="top" width="300">
12	<h3><A href="../../../index.htm"><IMG height="86" alt="C++ Boost" src="../../../boost.png" width="277" border="0"></A></h3>
13	</td>
14	<TD width="353">
15	<H1 align="center">Boost.Regex</H1>
16	<H2 align="center">
17	Perl Regular Expression Syntax</H2>
18	</TD>
19	<td width="50">
20	<h3><A href="index.html"><IMG height="45" alt="Boost.Regex Index" src="uarrow.gif" width="43" border="0"></A></h3>
21	</td>
22	</TR>
23	</TABLE>
24	</P>
25	<HR>
26	<H3>Contents</H3>
27	<dl class="index">
28	<dt><A href="#synopsis">Synopsis</A> <dt><A href="#Perl">Perl Syntax</A> <dt><A href="#what">
29	What Gets Matched</A> <dt><A href="#variations">Variations</A>
30	<dd>
31	<dt><A href="#options">Options</A> <dt><A href="#mods">Modifiers</A> <dt><A href="#refs">References</A></dt>
32	</dl>
33	<H3><A name="synopsis"></A>Synopsis</H3>
34	<P>The Perl regular expression syntax is based on that used by the programming
35	language <EM>Perl</EM> .  Perl regular expressions are the default
36	behavior in Boost.Regex or you can pass the flag <EM>perl</EM> to the
37	regex constructor, for example:</P>
38	<PRE>// e1 is a case sensitive Perl regular expression:
39	// since Perl is the default option there's no need to explicitly specify the syntax used here:
40	boost::regex e1(my_expression);
41	// e2 a case insensitive Perl regular expression:
42	boost::regex e2(my_expression, boost::regex::perl\|boost::regex::icase);</PRE>
43	<H3>Perl Regular Expression Syntax<A name="Perl"></A></H3>
44	<P>In Perl regular expressions, all characters match themselves except for
45	the following special characters:</P>
46	<PRE>.[{()\*+?\|^$</PRE>
47	<H4>Wildcard:</H4>
48	<P>The single character '.' when used outside of a character set will match any
49	single character except:</P>
50	<P>The NULL character when the flag <EM>match_no_dot_null</EM> is passed to the
51	matching algorithms.</P>
52	<P>The newline character when the flag <EM>match_not_dot_newline</EM> is passed to
53	the matching algorithms.</P>
54	<H4>Anchors:</H4>
55	<P>A '^' character shall match the start of a line.</P>
56	<P>A '$' character shall match the end of a line.</P>
57	<H4>Marked sub-expressions:</H4>
58	<P>A section beginning ( and ending ) acts as a marked sub-expression.
59	Whatever matched the sub-expression is split out in a separate field by the
60	matching algorithms.  Marked sub-expressions can also repeated, or
61	referred to by a back-reference.</P>
62	<H4>Non-marking grouping:</H4>
63	<P>A marked sub-expression is useful to lexically group part of a regular
64	expression, but has the side-effect of spitting out an extra field in the
65	result.  As an alternative you can lexically group part of a regular
66	expression, without generating a marked sub-expression by using (?: and ) , for
67	example (?:ab)+ will repeat "ab" without splitting out any separate
68	sub-expressions.</P>
69	<H4>Repeats:</H4>
70	<P>Any atom (a single character, a marked sub-expression, or a character class)
71	can be repeated with the *, +, ?, and {} operators.</P>
72	<P>The * operator will match the preceding atom zero or more times, for example
73	the expression a*b will match any of the following:</P>
74	<PRE>b
75	ab
76	aaaaaaaab</PRE>
77	<P>The + operator will match the preceding atom one or more times, for example the
78	expression a+b will match any of the following:</P>
79	<PRE>ab
80	aaaaaaaab</PRE>
81	<P>But will not match:</P>
82	<PRE>b</PRE>
83	<P>The ? operator will match the preceding atom zero or one times, for
84	example the expression ca?b will match any of the following:</P>
85	<PRE>cb
86	cab</PRE>
87	<P>But will not match:</P>
88	<PRE>caab</PRE>
89	<P>An atom can also be repeated with a bounded repeat:</P>
90	<P>a{n}  Matches 'a' repeated exactly <EM>n</EM> times.</P>
91	<P>a{n,}  Matches 'a' repeated <EM>n</EM> or more times.</P>
92	<P>a{n, m}  Matches 'a' repeated between <EM>n</EM> and <EM>m</EM> times
93	inclusive.</P>
94	<P>For example:</P>
95	<PRE>^a{2,3}$</PRE>
96	<P>Will match either of:</P>
97	<PRE>aa
98	aaa</PRE>
99	<P>But neither of:</P>
100	<PRE>a
101	aaaa</PRE>
102	<P>It is an error to use a repeat operator, if the preceding construct can not be
103	repeated, for example:</P>
104	<PRE>a(*)</PRE>
105	<P>Will raise an error, as there is nothing for the * operator to be applied to.</P>
106	<H4>Non greedy repeats</H4>
107	<P>The normal repeat operators are "greedy", that is to say they will consume as
108	much input as possible.  There are non-greedy versions available that will
109	consume as little input as possible while still producing a match.</P>
110	<P>*? Matches the previous atom zero or more times, while consuming as little
111	input as possible.</P>
112	<P>+? Matches the previous atom one or more times, while consuming as little input
113	as possible.</P>
114	<P>?? Matches the previous atom zero or one times, while consuming as little input
115	as possible.</P>
116	<P>{n,}? Matches the previous atom <EM>n</EM> or more times, while consuming
117	as little input as possible.</P>
118	<P>{n,m}? Matches the previous atom between <EM>n</EM> and <EM>m</EM> times,
119	while consuming as little input as possible.</P>
120	<H4>Back references:</H4>
121	<P>An escape character followed by a digit <EM>n</EM>, where <EM>n </EM>is in the
122	range 1-9, matches the same string that was matched by sub-expression <EM>n</EM>.
123	For example the expression:</P>
124	<PRE>^(a).\1$</PRE>
125	<P>Will match the string:</P>
126	<PRE>aaabbaaa</PRE>
127	<P>But not the string:</P>
128	<PRE>aaabba</PRE>
129	<H4>Alternation</H4>
130	<P>The \| operator will match either of its arguments, so for example: abc\|def will
131	match either "abc" or "def".
132	</P>
133	<P>Parenthesis can be used to group alternations, for example: ab(d\|ef) will match
134	either of "abd" or "abef".</P>
135	<P>Empty alternatives are not allowed (these are almost always a mistake),
136	but if you really want an empty alternative use (?:) as a placeholder, for
137	example:</P>
138	<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
139	<P>"\|abc" is not a valid expression, but<BR>
140	"(?:)\|abc" is and is equivalent, also the expression:<BR>
141	"(?:abc)??" has exactly the same effect.</P>
142	</BLOCKQUOTE>
143	<H4>Character sets:</H4>
144	<P>A character set is a bracket-expression starting with [ and ending with ], it
145	defines a set of characters, and matches any single character that is a member
146	of that set.</P>
147	<P>A bracket expression may contain any combination of the following:</P>
148	<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
149	<H5>Single characters:</H5>
150	<P>For example [abc], will match any of the characters 'a', 'b', or 'c'.</P>
151	<H5>Character ranges:</H5>
152	<P>For example [a-c] will match any single character in the range 'a' to
153	'c'.  By default, for POSIX-Perl regular expressions, a character <EM>x</EM>
154	is within the range <EM>y</EM> to <EM>z</EM>, if it collates within that
155	range; this results in locale specific behavior.  This behavior can
156	be turned off by unsetting the <EM><A href="syntax_option_type.html#Perl">collate</A></EM>
157	option flag - in which case whether a character appears within a range is
158	determined by comparing the code points of the characters only</P>
159	<H5>Negation:</H5>
160	<P>If the bracket-expression begins with the ^ character, then it matches the
161	complement of the characters it contains, for example [^a-c] matches any
162	character that is not in the range a-c.</P>
163	<H5>Character classes:</H5>
164	<P>An expression of the form [[:name:]] matches the named character class "name",
165	for example [[:lower:]] matches any lower case character.  See <A href="character_class_names.html">
166	character class names</A>.</P>
167	<H5>Collating Elements:</H5>
168	<P>An expression of the form [[.col.] matches the collating element <EM>col</EM>.
169	A collating element is any single character, or any sequence of characters that
170	collates as a single unit.  Collating elements may also be used as the end
171	point of a range, for example: [[.ae.]-c] matches the character sequence "ae",
172	plus any single character in the range "ae"-c, assuming that "ae" is treated as
173	a single collating element in the current locale.</P>
174	<P>As an extension, a collating element may also be specified via it's <A href="collating_names.html">
175	symbolic name</A>, for example:</P>
176	<P>[[.NUL.]]</P>
177	<P>matches a NUL character.</P>
178	<H5>Equivalence classes:</H5>
179	<P>
180	An expression oftheform[[=col=]], matches any character or collating element
181	whose primary sort key is the same as that for collating element <EM>col</EM>,
182	as with colating elements the name <EM>col</EM> may be a <A href="collating_names.html">
183	symbolic name</A>.  A primary sort key is one that ignores case,
184	accentation, or locale-specific tailorings; so for example [[=a=]] matches any
185	of the characters: a, à, á, â, ã, ä, å, A, À, Á, Â, Ã, Ä and Å.
186	Unfortunately implementation of this is reliant on the platform's collation and
187	localisation support; this feature can not be relied upon to work portably
188	across all platforms, or even all locales on one platform.</P>
189	<H5>Escapes:</H5>
190	<P>All the escape sequences that match a single character, or a single character
191	class are permitted within a character class definition, <EM>except</EM> the
192	negated character classes (\D \W etc).</P>
193	</BLOCKQUOTE>
194	<H5>Combinations:</H5>
195	<P>All of the above can be combined in one character set declaration, for example:
196	[[:digit:]a-c[.NUL.]].</P>
197	<H4>Escapes</H4>
198	<P>Any special character preceded by an escape shall match itself.
199	</P>
200	<P>The following escape sequences are also supported:</P>
201	<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
202	<H5>Escapes matching a specific character</H5>
203	<P>The following escape sequences are all synonyms for single characters:</P>
204	<P>
205	<TABLE id="Table7" cellSpacing="1" cellPadding="1" width="100%" border="1">
206	<TR>
207	<TD><STRONG>Escape</STRONG></TD>
208	<TD><STRONG>Character</STRONG></TD>
209	</TR>
210	<TR>
211	<TD>\a</TD>
212	<TD>'\a'</TD>
213	</TR>
214	<TR>
215	<TD>\e</TD>
216	<TD>0x1B</TD>
217	</TR>
218	<TR>
219	<TD>\f</TD>
220	<TD>\f</TD>
221	</TR>
222	<TR>
223	<TD>\n</TD>
224	<TD>\n</TD>
225	</TR>
226	<TR>
227	<TD>\r</TD>
228	<TD>\r</TD>
229	</TR>
230	<TR>
231	<TD>\t</TD>
232	<TD>\t</TD>
233	</TR>
234	<TR>
235	<TD>\v</TD>
236	<TD>\v</TD>
237	</TR>
238	<TR>
239	<TD>\b</TD>
240	<TD>\b (but only inside a character class declaration).</TD>
241	</TR>
242	<TR>
243	<TD>\cX</TD>
244	<TD>An ASCII escape sequence - the character whose code point is X % 32</TD>
245	</TR>
246	<TR>
247	<TD>\xdd</TD>
248	<TD>A hexadecimal escape sequence - matches the single character whose code point
249	is 0xdd.</TD>
250	</TR>
251	<TR>
252	<TD>\x{dddd}</TD>
253	<TD>A hexadecimal escape sequence - matches the single character whose code point
254	is 0xdddd.</TD>
255	</TR>
256	<TR>
257	<TD>\0ddd</TD>
258	<TD>An octal escape sequence - matches the single character whose code point is
259	0ddd.</TD>
260	</TR>
261	<TR>
262	<TD>\N{name}</TD>
263	<TD>Matches the single character which has the <A href="collating_names.html">symbolic
264	name</A> <EM>name.  </EM>For example \N{newline} matches the single
265	character \n.</TD>
266	</TR>
267	</TABLE>
268	</P>
269	<H5>"Single character" character classes:</H5>
270	<P>Any escaped character <EM>x</EM>, if <EM>x</EM> is the name of a character
271	class shall match any character that is a member of that class, and any escaped
272	character <EM>X</EM>, if <EM>x</EM> is the name of a character class, shall
273	match any character not in that class.</P>
274	<P>The following are supported by default:</P>
275	<P>
276	<TABLE id="Table3" cellSpacing="1" cellPadding="1" width="300" border="1">
277	<TR>
278	<TD><STRONG>Escape sequence</STRONG></TD>
279	<TD><STRONG>Equivalent to</STRONG></TD>
280	</TR>
281	<TR>
282	<TD>\d</TD>
283	<TD>[[:digit:]]</TD>
284	</TR>
285	<TR>
286	<TD>\l</TD>
287	<TD>[[:lower:]]</TD>
288	</TR>
289	<TR>
290	<TD>\s</TD>
291	<TD>[[:space:]]</TD>
292	</TR>
293	<TR>
294	<TD>\u</TD>
295	<TD>[[:upper:]]</TD>
296	</TR>
297	<TR>
298	<TD>\w</TD>
299	<TD>[[:word:]]</TD>
300	</TR>
301	<TR>
302	<TD>\D</TD>
303	<TD>[^[:digit:]]</TD>
304	</TR>
305	<TR>
306	<TD>\L</TD>
307	<TD>[^[:lower:]]</TD>
308	</TR>
309	<TR>
310	<TD>\S</TD>
311	<TD>[^[:space:]]</TD>
312	</TR>
313	<TR>
314	<TD>\U</TD>
315	<TD>[^[:upper:]]</TD>
316	</TR>
317	<TR>
318	<TD>\W</TD>
319	<TD>[^[:word:]]</TD>
320	</TR>
321	</TABLE>
322	</P>
323	<H5>Character Properties</H5>
324	<P>The character property names in the following table are all equivalent to the <A href="character_class_names.html">
325	names used in character classes</A>.</P>
326	<P>
327	<TABLE id="Table9" cellSpacing="1" cellPadding="1" width="100%" border="0">
328	<TR>
329	<TD><STRONG>Form</STRONG></TD>
330	<TD><STRONG>Description</STRONG></TD>
331	<TD><STRONG>Equivalent character set form</STRONG></TD>
332	</TR>
333	<TR>
334	<TD>\pX</TD>
335	<TD>Matches any character that has the property X.</TD>
336	<TD>[[:X:]]</TD>
337	</TR>
338	<TR>
339	<TD>\p{Name}</TD>
340	<TD>Matches any character that has the property <EM>Name</EM>.</TD>
341	<TD>[[:Name:]]</TD>
342	</TR>
343	<TR>
344	<TD>\PX</TD>
345	<TD>Matches any character that does not have the property X.</TD>
346	<TD>[^[:X:]]</TD>
347	</TR>
348	<TR>
349	<TD>\P{Name}</TD>
350	<TD>Matches any character that does not have the property <EM>Name</EM>.</TD>
351	<TD>[^[:Name:]]</TD>
352	</TR>
353	</TABLE>
354	</P>
355	<H5>Word Boundaries</H5>
356	<P>The following escape sequences match the boundaries of words:</P>
357	<P>
358	<TABLE id="Table4" cellSpacing="1" cellPadding="1" width="100%" border="1">
359	<TR>
360	<TD>\<</TD>
361	<TD>Matches the start of a word.</TD>
362	</TR>
363	<TR>
364	<TD>\></TD>
365	<TD>Matches the end of a word.</TD>
366	</TR>
367	<TR>
368	<TD>\b</TD>
369	<TD>Matches a word boundary (the start or end of a word).</TD>
370	</TR>
371	<TR>
372	<TD>\B</TD>
373	<TD>Matches only when not at a word boundary.</TD>
374	</TR>
375	</TABLE>
376	</P>
377	<H5>Buffer boundaries</H5>
378	<P>The following match only at buffer boundaries: a "buffer" in this context is
379	the whole of the input text that is being matched against (note that ^ and
380	$ may match embedded newlines within the text).</P>
381	<P>
382	<TABLE id="Table5" cellSpacing="1" cellPadding="1" width="100%" border="1">
383	<TR>
384	<TD>\`</TD>
385	<TD>Matches at the start of a buffer only.</TD>
386	</TR>
387	<TR>
388	<TD>\'</TD>
389	<TD>Matches at the end of a buffer only.</TD>
390	</TR>
391	<TR>
392	<TD>\A</TD>
393	<TD>Matches at the start of a buffer only (the same as \`).</TD>
394	</TR>
395	<TR>
396	<TD>\z</TD>
397	<TD>Matches at the end of a buffer only (the same as \').</TD>
398	</TR>
399	<TR>
400	<TD>\Z</TD>
401	<TD>Matches an optional sequence of newlines at the end of a buffer: equivalent to
402	the regular expression \n*\z</TD>
403	</TR>
404	</TABLE>
405	</P>
406	<H5>Continuation Escape</H5>
407	<P>The sequence \G matches only at the end of the last match found, or at the
408	start of the text being matched if no previous match was found.  This
409	escape useful if you're iterating over the matches contained within a text, and
410	you want each subsequence match to start where the last one ended.</P>
411	<H5>Quoting escape</H5>
412	<P>The escape sequence \Q begins a "quoted sequence": all the subsequent
413	characters are treated as literals, until either the end of the regular
414	expression or \E is found.  For example the expression: \Q\*+\Ea+ would
415	match either of:</P>
416	<PRE>\+a<BR>\+aaa</PRE>
417	<H5>Unicode escapes</H5>
418	<P>
419	<TABLE id="Table6" cellSpacing="1" cellPadding="1" width="100%" border="1">
420	<TR>
421	<TD>\C</TD>
422	<TD>Matches a single code point: in Boost regex this has exactly the same effect
423	as a "." operator.</TD>
424	</TR>
425	<TR>
426	<TD>\X</TD>
427	<TD>Matches a combining character sequence: that is any non-combining character
428	followed by a sequence of zero or more combining characters.</TD>
429	</TR>
430	</TABLE>
431	</P>
432	<H5>Any other escape</H5>
433	<P>Any other escape sequence matches the character that is escaped, for example \@
434	matches a literal <A href="mailto:'@'">'@'</A>.</P>
435	</BLOCKQUOTE>
436	<H4 dir="ltr">Perl Extended Patterns</H4>
437	<P dir="ltr">Perl-specific extensions to the regular expression syntax all start
438	with (?.</P>
439	<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
440	<H5 dir="ltr">Comments</H5>
441	<P dir="ltr">(?# ... ) is treated as a comment, it's contents are ignored.</P>
442	<H5 dir="ltr">Modifiers</H5>
443	<P dir="ltr">(?imsx-imsx ... ) alters which of the perl modifiers are in effect
444	within the pattern, changes take effect from the point that the block is first
445	seen and extend to any enclosing ).  Letters before a '-' turn that perl
446	modifier on, letters afterward, turn it off.</P>
447	<P dir="ltr">(?imsx-imsx:pattern) applies the specified modifiers to <EM>pattern</EM>
448	only.</P>
449	<H5 dir="ltr">Non-marking grouping</H5>
450	<P dir="ltr">(?:pattern) lexically groups <EM>pattern</EM>, without generating an
451	additional sub-expression.</P>
452	<H5 dir="ltr">Lookahead</H5>
453	<P dir="ltr">(?=pattern) consumes zero characters, only if <EM>pattern</EM> matches.</P>
454	<P dir="ltr">(?!pattern) consumes zero characters, only if <EM>pattern</EM> does
455	not match.</P>
456	<P dir="ltr">Lookahead is typically used to create the logical AND of two regular
457	expressions, for example if a password must contain a lower case letter, an
458	upper case letter, a punctuation symbol, and be at least 6 characters long,
459	then the expression:</P>
460	<PRE dir="ltr">(?=.[[:lower:]])(?=.[[:upper:]])(?=.*[[:punct:]]).{6,}</PRE>
461	<P dir="ltr">could be used to validate the password.</P>
462	<H5 dir="ltr">Lookbehind</H5>
463	<P dir="ltr">(?<=pattern) consumes zero characters, only if <EM>pattern</EM> could
464	be matched against the characters preceding the current position (<EM>pattern</EM>
465	must be of fixed length).</P>
466	<P dir="ltr">(?<!pattern) consumes zero characters, only if <EM>pattern</EM> could
467	not be matched against the characters preceding the current position (<EM>pattern</EM>
468	must be of fixed length).</P>
469	<H5 dir="ltr">Independent sub-expressions</H5>
470	<P dir="ltr">(?>pattern) <EM>pattern</EM> is matched independently of the
471	surrounding patterns, the expression will never backtrack into <EM>pattern</EM>.
472	Independent sub-expressions are typically used to improve performance; only the
473	best possible match for <EM>pattern</EM> will be considered, if this doesn't
474	allow the expression as a whole to match then no match is found at all.</P>
475	<H5 dir="ltr">Conditional Expressions</H5>
476	<P dir="ltr">(?(condition)yes-pattern\|no-pattern) attempts to match <EM>yes-pattern</EM>
477	if the <EM>condition </EM>is true, otherwise attempts to match <EM>no-pattern</EM>.</P>
478	<P dir="ltr">(?(condition)yes-pattern) attempts to match <EM>yes-pattern</EM> if
479	the <EM>condition </EM>is true, otherwise fails.</P>
480	<P dir="ltr"><EM>Condition</EM> may be either a forward lookahead assert, or the
481	index of a marked sub-expression (the condition becomes true if the
482	sub-expression has been matched).</P>
483	</BLOCKQUOTE><A name="what">
484	<H4>Operator precedence</H4>
485	<P> The order of precedence for of operators is as shown in the following
486	table:</P>
487	<P>
488	<TABLE id="Table2" cellSpacing="1" cellPadding="1" width="100%" border="1">
489	<TR>
490	<TD>Collation-related bracket symbols</TD>
491	<TD>[==] [::] [..]</TD>
492	</TR>
493	<TR>
494	<TD>Escaped characters
495	</TD>
496	<TD>\</TD>
497	</TR>
498	<TR>
499	<TD>Character set (bracket expression)
500	</TD>
501	<TD>[]</TD>
502	</TR>
503	<TR>
504	<TD>Grouping</TD>
505	<TD>()</TD>
506	</TR>
507	<TR>
508	<TD>Single-character-ERE duplication
509	</TD>
510	<TD>* + ? {m,n}</TD>
511	</TR>
512	<TR>
513	<TD>Concatenation</TD>
514	<TD></TD>
515	</TR>
516	<TR>
517	<TD>Anchoring</TD>
518	<TD>^$</TD>
519	</TR>
520	<TR>
521	<TD>Alternation</TD>
522	<TD>\|</TD>
523	</TR>
524	</TABLE>
525	</P>
526	</A>
527	<H3>What gets matched</H3>
528	<P>If you view the regular expression as a directed (possibly cyclic) graph, then
529	the best match found is the first match found by a depth-first-search performed
530	on that graph, while matching the input text.</P>
531	<P>Alternatively:</P>
532	<P>the best match found is the leftmost match, with individual elements matched as
533	follows;</P>
534	<P>
535	<TABLE id="Table8" cellSpacing="1" cellPadding="1" width="100%" border="0">
536	<TR>
537	<TD><STRONG>Construct</STRONG></TD>
538	<TD><STRONG>What gets matches</STRONG></TD>
539	</TR>
540	<TR>
541	<TD>AtomA AtomB</TD>
542	<TD>Locates the best match for AtomA that has a following match for AtomB.</TD>
543	</TR>
544	<TR>
545	<TD>Expression1 \| Expression2</TD>
546	<TD>If Expresion1 can be matched then returns that match, otherwise attempts to
547	match Expression2.</TD>
548	</TR>
549	<TR>
550	<TD>S{N}</TD>
551	<TD>Matches S repeated exactly N times.</TD>
552	</TR>
553	<TR>
554	<TD>S{N,M}</TD>
555	<TD>Matches S repeated between N and M times, and as many times as possible.</TD>
556	</TR>
557	<TR>
558	<TD>S{N,M}?</TD>
559	<TD>Matches S repeated between N and M times, and as few times as possible.</TD>
560	</TR>
561	<TR>
562	<TD><!--StartFragment --> S?, S*, S+</TD>
563	<TD><!--StartFragment --> The same as <CODE>S{0,1}</CODE>, <CODE>S{0,UINT_MAX}</CODE>,
564	<CODE>S{1,UINT_MAX}</CODE> respectively.
565	</TD>
566	</TR>
567	<TR>
568	<TD>S??, S*?, S+?</TD>
569	<TD>The same as <CODE>S{0,1}?</CODE>, <CODE>S{0,UINT_MAX}?</CODE>, <CODE>S{1,UINT_MAX}?</CODE>
570	respectively.
571	</TD>
572	</TR>
573	<TR>
574	<TD><!--StartFragment --> (?>S)
575	</TD>
576	<TD>Matches the best match for S, and only that.</TD>
577	</TR>
578	<TR>
579	<TD>
580	(?=S), (?<=S)
581	</TD>
582	<TD>Matches only the best match for S (this is only visible if there are capturing
583	parenthesis within S).</TD>
584	</TR>
585	<TR>
586	<TD><!--StartFragment --> (?!S), (?<!S)</TD>
587	<TD>Considers only whether a match for S exists or not.</TD>
588	</TR>
589	<TR>
590	<TD><!--StartFragment --> (?(condition)yes-pattern \| no-pattern)</TD>
591	<TD>If condition is <EM>true</EM>, then only <EM>yes-pattern</EM> is considered,
592	otherwise only <EM>no-pattern</EM> is considered.</TD>
593	</TR>
594	</TABLE>
595	</P>
596	<H3><A name="variations"></A>Variations</H3>
597	<P>The options <A href="syntax_option_type.html#perl"><EM>normal, ECMAScript, JavaScript</EM>
598	and <EM>JScript</EM></A> are all synonyms for <EM>Perl</EM>.</P>
599	<H3><A name="options"></A>Options</H3>
600	<P>There are a <A href="syntax_option_type.html#Perl">variety of flags</A> that
601	may be combined with the <EM>Perl</EM> option when constructing the regular
602	expression, in particular note that the <A href="syntax_option_type.html#Perl">newline_alt</A>
603	option alters the syntax, while the <A href="syntax_option_type.html#Perl">collate,
604	nosubs and icase</A> options modify how the case and locale sensitivity
605	are to be applied.</P>
606	<H3><A name="mods"></A>Modifiers</H3>
607	<P>The perl <EM>smix</EM> modifiers can either be applied using a (?smix-smix)
608	prefix to the regular expression, or with one of the regex-compile time flags <EM><A href="syntax_option_type.html#Perl">
609	no_mod_m, mod_x, mod_s, and no_mod_s</A></EM>.
610	</P>
611	<H3><A name="refs">References</H3>
612	<P><A href="http://perldoc.perl.org/perlre.html"> Perl 5.8.</A></P>
613	<HR>
614	<P></P>
615	<p>Revised
616	<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
617	21 Aug 2004
618	<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
619	<P><I>© Copyright <a href="mailto:jm@regex.fsnet.co.uk">John Maddock</a> 2004</I></P>
620	<I>
621	<P><I>Use, modification and distribution are subject to the Boost Software License,
622	Version 1.0. (See accompanying file <A href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A>
623	or copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>).</I></P>
624	</I>
625	</body>
626	</html>

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