1REGCOMP(3P) POSIX Programmer's Manual REGCOMP(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 regcomp, regerror, regexec, regfree — regular expression matching
13
15 #include <regex.h>
16 int regcomp(regex_t *restrict preg, const char *restrict pattern,
18 int cflags);
19 size_t regerror(int errcode, const regex_t *restrict preg,
20 char *restrict errbuf, size_t errbuf_size);
21 int regexec(const regex_t *restrict preg, const char *restrict string,
22 size_t nmatch, regmatch_t pmatch[restrict], int eflags);
23 void regfree(regex_t *preg);
24
26 These functions interpret basic and extended regular expressions as
27 described in the Base Definitions volume of POSIX.1‐2017, Chapter 9,
28 Regular Expressions.
29 The regex_t structure is defined in <regex.h> and contains at least the
31 following member:
32 ┌──────────────┬──────────────┬───────────────────────────┐
34 │Member Type │ Member Name │ Description │
35 ├──────────────┼──────────────┼───────────────────────────┤
36 │size_t │re_nsub │ Number of parenthesized │
37 │ │ │ subexpressions. │
38 └──────────────┴──────────────┴───────────────────────────┘
39 The regmatch_t structure is defined in <regex.h> and contains at least
40 the following members:
41 ┌──────────────┬──────────────┬───────────────────────────┐
43 │Member Type │ Member Name │ Description │
44 ├──────────────┼──────────────┼───────────────────────────┤
45 │regoff_t │rm_so │ Byte offset from start of │
46 │ │ │ string to start of sub‐ │
47 │ │ │ string. │
48 │regoff_t │rm_eo │ Byte offset from start of │
49 │ │ │ string of the first char‐ │
50 │ │ │ acter after the end of │
51 │ │ │ substring. │
52 └──────────────┴──────────────┴───────────────────────────┘
53 The regcomp() function shall compile the regular expression contained
54 in the string pointed to by the pattern argument and place the results
55 in the structure pointed to by preg. The cflags argument is the bit‐
56 wise-inclusive OR of zero or more of the following flags, which are
57 defined in the <regex.h> header:
58 REG_EXTENDED Use Extended Regular Expressions.
60 REG_ICASE Ignore case in match (see the Base Definitions volume of
62 POSIX.1‐2017, Chapter 9, Regular Expressions).
63 REG_NOSUB Report only success/fail in regexec().
65 REG_NEWLINE Change the handling of <newline> characters, as described
67 in the text.
68 The default regular expression type for pattern is a Basic Regular
70 Expression. The application can specify Extended Regular Expressions
71 using the REG_EXTENDED cflags flag.
72 If the REG_NOSUB flag was not set in cflags, then regcomp() shall set
74 re_nsub to the number of parenthesized subexpressions (delimited by
75 "\(\)" in basic regular expressions or "()" in extended regular expres‐
76 sions) found in pattern.
77 The regexec() function compares the null-terminated string specified by
79 string with the compiled regular expression preg initialized by a pre‐
80 vious call to regcomp(). If it finds a match, regexec() shall return
81 0; otherwise, it shall return non-zero indicating either no match or an
82 error. The eflags argument is the bitwise-inclusive OR of zero or more
83 of the following flags, which are defined in the <regex.h> header:
84 REG_NOTBOL The first character of the string pointed to by string is
86 not the beginning of the line. Therefore, the <circum‐
87 flex> character ('^'), when taken as a special character,
88 shall not match the beginning of string.
89 REG_NOTEOL The last character of the string pointed to by string is
91 not the end of the line. Therefore, the <dollar-sign>
92 ('$'), when taken as a special character, shall not match
93 the end of string.
94 If nmatch is 0 or REG_NOSUB was set in the cflags argument to reg‐
96 comp(), then regexec() shall ignore the pmatch argument. Otherwise, the
97 application shall ensure that the pmatch argument points to an array
98 with at least nmatch elements, and regexec() shall fill in the elements
99 of that array with offsets of the substrings of string that correspond
100 to the parenthesized subexpressions of pattern: pmatch[i].rm_so shall
101 be the byte offset of the beginning and pmatch[i].rm_eo shall be one
102 greater than the byte offset of the end of substring i. (Subexpression
103 i begins at the ith matched open parenthesis, counting from 1.) Offsets
104 in pmatch[0] identify the substring that corresponds to the entire reg‐
105 ular expression. Unused elements of pmatch up to pmatch[nmatch-1] shall
106 be filled with -1. If there are more than nmatch subexpressions in pat‐
107 tern (pattern itself counts as a subexpression), then regexec() shall
108 still do the match, but shall record only the first nmatch substrings.
109 When matching a basic or extended regular expression, any given paren‐
111 thesized subexpression of pattern might participate in the match of
112 several different substrings of string, or it might not match any sub‐
113 string even though the pattern as a whole did match. The following
114 rules shall be used to determine which substrings to report in pmatch
115 when matching regular expressions:
116 1. If subexpression i in a regular expression is not contained within
118 another subexpression, and it participated in the match several
119 times, then the byte offsets in pmatch[i] shall delimit the last
120 such match.
121 2. If subexpression i is not contained within another subexpression,
123 and it did not participate in an otherwise successful match, the
124 byte offsets in pmatch[i] shall be -1. A subexpression does not
125 participate in the match when:
126 '*' or "\{\}" appears immediately after the subexpression in a
128 basic regular expression, or '*', '?', or "{}" appears immediately
129 after the subexpression in an extended regular expression, and the
130 subexpression did not match (matched 0 times)
131 or:
133 '|' is used in an extended regular expression to select this
135 subexpression or another, and the other subexpression
136 matched.
137 3. If subexpression i is contained within another subexpression j, and
139 i is not contained within any other subexpression that is contained
140 within j, and a match of subexpression j is reported in pmatch[j],
141 then the match or non-match of subexpression i reported in
142 pmatch[i] shall be as described in 1. and 2. above, but within the
143 substring reported in pmatch[j] rather than the whole string. The
144 offsets in pmatch[i] are still relative to the start of string.
145 4. If subexpression i is contained in subexpression j, and the byte
147 offsets in pmatch[j] are -1, then the pointers in pmatch[i] shall
148 also be -1.
149 5. If subexpression i matched a zero-length string, then both byte
151 offsets in pmatch[i] shall be the byte offset of the character or
152 null terminator immediately following the zero-length string.
153 If, when regexec() is called, the locale is different from when the
155 regular expression was compiled, the result is undefined.
156 If REG_NEWLINE is not set in cflags, then a <newline> in pattern or
158 string shall be treated as an ordinary character. If REG_NEWLINE is
159 set, then <newline> shall be treated as an ordinary character except as
160 follows:
161 1. A <newline> in string shall not be matched by a <period> outside a
163 bracket expression or by any form of a non-matching list (see the
164 Base Definitions volume of POSIX.1‐2017, Chapter 9, Regular Expres‐
165 sions).
166 2. A <circumflex> ('^') in pattern, when used to specify expression
168 anchoring (see the Base Definitions volume of POSIX.1‐2017, Section
169 9.3.8, BRE Expression Anchoring), shall match the zero-length
170 string immediately after a <newline> in string, regardless of the
171 setting of REG_NOTBOL.
172 3. A <dollar-sign> ('$') in pattern, when used to specify expression
174 anchoring, shall match the zero-length string immediately before a
175 <newline> in string, regardless of the setting of REG_NOTEOL.
176 The regfree() function frees any memory allocated by regcomp() associ‐
178 ated with preg.
179 The following constants are defined as the minimum set of error return
181 values, although other errors listed as implementation extensions in
182 <regex.h> are possible:
183 REG_BADBR Content of "\{\}" invalid: not a number, number too
185 large, more than two numbers, first larger than second.
186 REG_BADPAT Invalid regular expression.
188 REG_BADRPT '?', '*', or '+' not preceded by valid regular expres‐
190 sion.
191 REG_EBRACE "\{\}" imbalance.
193 REG_EBRACK "[]" imbalance.
195 REG_ECOLLATE Invalid collating element referenced.
197 REG_ECTYPE Invalid character class type referenced.
199 REG_EESCAPE Trailing <backslash> character in pattern.
201 REG_EPAREN "\(\)" or "()" imbalance.
203 REG_ERANGE Invalid endpoint in range expression.
205 REG_ESPACE Out of memory.
207 REG_ESUBREG Number in "\digit" invalid or in error.
209 REG_NOMATCH regexec() failed to match.
211 If more than one error occurs in processing a function call, any one of
213 the possible constants may be returned, as the order of detection is
214 unspecified.
215 The regerror() function provides a mapping from error codes returned by
217 regcomp() and regexec() to unspecified printable strings. It generates
218 a string corresponding to the value of the errcode argument, which the
219 application shall ensure is the last non-zero value returned by reg‐
220 comp() or regexec() with the given value of preg. If errcode is not
221 such a value, the content of the generated string is unspecified.
222 If preg is a null pointer, but errcode is a value returned by a previ‐
224 ous call to regexec() or regcomp(), the regerror() still generates an
225 error string corresponding to the value of errcode, but it might not be
226 as detailed under some implementations.
227 If the errbuf_size argument is not 0, regerror() shall place the gener‐
229 ated string into the buffer of size errbuf_size bytes pointed to by
230 errbuf. If the string (including the terminating null) cannot fit in
231 the buffer, regerror() shall truncate the string and null-terminate the
232 result.
233 If errbuf_size is 0, regerror() shall ignore the errbuf argument, and
235 return the size of the buffer needed to hold the generated string.
236 If the preg argument to regexec() or regfree() is not a compiled regu‐
238 lar expression returned by regcomp(), the result is undefined. A preg
239 is no longer treated as a compiled regular expression after it is given
240 to regfree().
241
243 Upon successful completion, the regcomp() function shall return 0. Oth‐
244 erwise, it shall return an integer value indicating an error as
245 described in <regex.h>, and the content of preg is undefined. If a code
246 is returned, the interpretation shall be as given in <regex.h>.
247 If regcomp() detects an invalid RE, it may return REG_BADPAT, or it may
249 return one of the error codes that more precisely describes the error.
250 Upon successful completion, the regexec() function shall return 0. Oth‐
252 erwise, it shall return REG_NOMATCH to indicate no match.
253 Upon successful completion, the regerror() function shall return the
255 number of bytes needed to hold the entire generated string, including
256 the null termination. If the return value is greater than errbuf_size,
257 the string returned in the buffer pointed to by errbuf has been trun‐
258 cated.
259 The regfree() function shall not return a value.
261
263 No errors are defined.
264 The following sections are informative.
266
268 #include <regex.h>
269 /*
271 * Match string against the extended regular expression in
272 * pattern, treating errors as no match.
273 *
274 * Return 1 for match, 0 for no match.
275 */
276 int
278 match(const char *string, char *pattern)
279 {
280 int status;
281 regex_t re;
282 if (regcomp(&re, pattern, REG_EXTENDED|REG_NOSUB) != 0) {
284 return(0); /* Report error. */
285 }
286 status = regexec(&re, string, (size_t) 0, NULL, 0);
287 regfree(&re);
288 if (status != 0) {
289 return(0); /* Report error. */
290 }
291 return(1);
292 }
293 The following demonstrates how the REG_NOTBOL flag could be used with
295 regexec() to find all substrings in a line that match a pattern sup‐
296 plied by a user. (For simplicity of the example, very little error
297 checking is done.)
298 (void) regcomp (&re, pattern, 0);
301 /* This call to regexec() finds the first match on the line. */
302 error = regexec (&re, &buffer[0], 1, &pm, 0);
303 while (error == 0) { /* While matches found. */
304 /* Substring found between pm.rm_so and pm.rm_eo. */
305 /* This call to regexec() finds the next match. */
306 error = regexec (&re, buffer + pm.rm_eo, 1, &pm, REG_NOTBOL);
307 }
308
310 An application could use:
311 regerror(code,preg,(char *)NULL,(size_t)0)
314 to find out how big a buffer is needed for the generated string, mal‐
316 loc() a buffer to hold the string, and then call regerror() again to
317 get the string. Alternatively, it could allocate a fixed, static buffer
318 that is big enough to hold most strings, and then use malloc() to allo‐
319 cate a larger buffer if it finds that this is too small.
320 To match a pattern as described in the Shell and Utilities volume of
322 POSIX.1‐2017, Section 2.13, Pattern Matching Notation, use the
323 fnmatch() function.
324
326 The regexec() function must fill in all nmatch elements of pmatch,
327 where nmatch and pmatch are supplied by the application, even if some
328 elements of pmatch do not correspond to subexpressions in pattern. The
329 application developer should note that there is probably no reason for
330 using a value of nmatch that is larger than preg->re_nsub+1.
331 The REG_NEWLINE flag supports a use of RE matching that is needed in
333 some applications like text editors. In such applications, the user
334 supplies an RE asking the application to find a line that matches the
335 given expression. An anchor in such an RE anchors at the beginning or
336 end of any line. Such an application can pass a sequence of <new‐
337 line>-separated lines to regexec() as a single long string and specify
338 REG_NEWLINE to regcomp() to get the desired behavior. The application
339 must ensure that there are no explicit <newline> characters in pattern
340 if it wants to ensure that any match occurs entirely within a single
341 line.
342 The REG_NEWLINE flag affects the behavior of regexec(), but it is in
344 the cflags parameter to regcomp() to allow flexibility of implementa‐
345 tion. Some implementations will want to generate the same compiled RE
346 in regcomp() regardless of the setting of REG_NEWLINE and have
347 regexec() handle anchors differently based on the setting of the flag.
348 Other implementations will generate different compiled REs based on the
349 REG_NEWLINE.
350 The REG_ICASE flag supports the operations taken by the grep -i option
352 and the historical implementations of ex and vi. Including this flag
353 will make it easier for application code to be written that does the
354 same thing as these utilities.
355 The substrings reported in pmatch[] are defined using offsets from the
357 start of the string rather than pointers. This allows type-safe access
358 to both constant and non-constant strings.
359 The type regoff_t is used for the elements of pmatch[] to ensure that
361 the application can represent large arrays in memory (important for an
362 application conforming to the Shell and Utilities volume of
363 POSIX.1‐2017).
364 The 1992 edition of this standard required regoff_t to be at least as
366 wide as off_t, to facilitate future extensions in which the string to
367 be searched is taken from a file. However, these future extensions have
368 not appeared. The requirement rules out popular implementations with
369 32-bit regoff_t and 64-bit off_t, so it has been removed.
370 The standard developers rejected the inclusion of a regsub() function
372 that would be used to do substitutions for a matched RE. While such a
373 routine would be useful to some applications, its utility would be much
374 more limited than the matching function described here. Both RE parsing
375 and substitution are possible to implement without support other than
376 that required by the ISO C standard, but matching is much more complex
377 than substituting. The only difficult part of substitution, given the
378 information supplied by regexec(), is finding the next character in a
379 string when there can be multi-byte characters. That is a much larger
380 issue, and one that needs a more general solution.
381 The errno variable has not been used for error returns to avoid filling
383 the errno name space for this feature.
384 The interface is defined so that the matched substrings rm_sp and rm_ep
386 are in a separate regmatch_t structure instead of in regex_t. This
387 allows a single compiled RE to be used simultaneously in several con‐
388 texts; in main() and a signal handler, perhaps, or in multiple threads
389 of lightweight processes. (The preg argument to regexec() is declared
390 with type const, so the implementation is not permitted to use the
391 structure to store intermediate results.) It also allows an application
392 to request an arbitrary number of substrings from an RE. The number of
393 subexpressions in the RE is reported in re_nsub in preg. With this
394 change to regexec(), consideration was given to dropping the REG_NOSUB
395 flag since the user can now specify this with a zero nmatch argument to
396 regexec(). However, keeping REG_NOSUB allows an implementation to use
397 a different (perhaps more efficient) algorithm if it knows in regcomp()
398 that no subexpressions need be reported. The implementation is only
399 required to fill in pmatch if nmatch is not zero and if REG_NOSUB is
400 not specified. Note that the size_t type, as defined in the ISO C stan‐
401 dard, is unsigned, so the description of regexec() does not need to
402 address negative values of nmatch.
403 REG_NOTBOL was added to allow an application to do repeated searches
405 for the same pattern in a line. If the pattern contains a <circumflex>
406 character that should match the beginning of a line, then the pattern
407 should only match when matched against the beginning of the line.
408 Without the REG_NOTBOL flag, the application could rewrite the expres‐
409 sion for subsequent matches, but in the general case this would require
410 parsing the expression. The need for REG_NOTEOL is not as clear; it was
411 added for symmetry.
412 The addition of the regerror() function addresses the historical need
414 for conforming application programs to have access to error information
415 more than ``Function failed to compile/match your RE for unknown rea‐
416 sons''.
417 This interface provides for two different methods of dealing with error
419 conditions. The specific error codes (REG_EBRACE, for example), defined
420 in <regex.h>, allow an application to recover from an error if it is so
421 able. Many applications, especially those that use patterns supplied by
422 a user, will not try to deal with specific error cases, but will just
423 use regerror() to obtain a human-readable error message to present to
424 the user.
425 The regerror() function uses a scheme similar to confstr() to deal with
427 the problem of allocating memory to hold the generated string. The
428 scheme used by strerror() in the ISO C standard was considered unac‐
429 ceptable since it creates difficulties for multi-threaded applications.
430 The preg argument is provided to regerror() to allow an implementation
432 to generate a more descriptive message than would be possible with
433 errcode alone. An implementation might, for example, save the character
434 offset of the offending character of the pattern in a field of preg,
435 and then include that in the generated message string. The implementa‐
436 tion may also ignore preg.
437 A REG_FILENAME flag was considered, but omitted. This flag caused
439 regexec() to match patterns as described in the Shell and Utilities
440 volume of POSIX.1‐2017, Section 2.13, Pattern Matching Notation instead
441 of REs. This service is now provided by the fnmatch() function.
442 Notice that there is a difference in philosophy between the
444 ISO POSIX‐2:1993 standard and POSIX.1‐2008 in how to handle a ``bad''
445 regular expression. The ISO POSIX‐2:1993 standard says that many bad
446 constructs ``produce undefined results'', or that ``the interpretation
447 is undefined''. POSIX.1‐2008, however, says that the interpretation of
448 such REs is unspecified. The term ``undefined'' means that the action
449 by the application is an error, of similar severity to passing a bad
450 pointer to a function.
451 The regcomp() and regexec() functions are required to accept any null-
453 terminated string as the pattern argument. If the meaning of the string
454 is ``undefined'', the behavior of the function is ``unspecified''.
455 POSIX.1‐2008 does not specify how the functions will interpret the pat‐
456 tern; they might return error codes, or they might do pattern matching
457 in some completely unexpected way, but they should not do something
458 like abort the process.
459
461 None.
462
464 fnmatch(), glob()
465 The Base Definitions volume of POSIX.1‐2017, Chapter 9, Regular Expres‐
467 sions, <regex.h>, <sys_types.h>
468 The Shell and Utilities volume of POSIX.1‐2017, Section 2.13, Pattern
470 Matching Notation
471
473 Portions of this text are reprinted and reproduced in electronic form
474 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
475 table Operating System Interface (POSIX), The Open Group Base Specifi‐
476 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
477 Electrical and Electronics Engineers, Inc and The Open Group. In the
478 event of any discrepancy between this version and the original IEEE and
479 The Open Group Standard, the original IEEE and The Open Group Standard
480 is the referee document. The original Standard can be obtained online
481 at http://www.opengroup.org/unix/online.html .
482 Any typographical or formatting errors that appear in this page are
484 most likely to have been introduced during the conversion of the source
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487IEEE/The Open Group 2017 REGCOMP(3P)