POSIX C正则库
POSIX C正则库
****2011年 02月 08日 星期二 00:35:38 CST
标准C库中没有正则表达式相关的函数,一般来说C中使用两种正则表达式库,一为POSIX C正则库,二为perl正则库PCRE。相比较而言PCRE要强大些,POSIX C正则库就足够使用。
标准C库中没有正则表达式相关的函数,一般来说C中使用两种正则表达式库,一为POSIX C正则库,二为perl正则库PCRE。相比较而言PCRE要强大些,POSIX C正则库就足够使用。
POSIX C正则库很好使用,一共只需掌握4个函数(regcomp, regexec, regerror, regfree)的使用即可,在 UNIX/Linux下最好的材料当然是man page,可以使用命令man regex查看POSIX regex functions,本文的最后附一份此man page。或者是在Linux下使用更详细的info page,C-s regex查找到* regcomp: (libc)POSIX Regexp Compilation.节,info page更详细些,在讲regerror节甚至给出了一个一般情况下使用的函数char *get_regerror (int errcode, regex_t *compiled);
- regcomp
该函数的原型如下:
int regcomp(regex_t *preg, const char *regex,int cflags);
POSIX C正则库为了提高效率,在将一个字符串与正则表达式进行比较之前,首先要用regcomp()函数对它进行编译,将其转化为regex_t类型,可以在/usr/include/regex.h中看到该类型是结构struct re_pattern_buffer的typedef。regex字符形式的正则表达式串。
cflags有以下几种:
cflags有以下几种:
- REG_EXTENDED 启用POSIX正则库扩展,关于该扩展的详细信息可参考POSIX规范。
- REG_ICASE 忽略大小写,相当于grep的-i参数。
- REG_NOSUB 不要存储子表达式
- REG_NEWLINE 把换行符作为多行的分隔符,这样'$'可匹配每一行的行尾,'^'匹配每一行的行首,'.'不匹配换行符,[^...]不匹配新行
- regexec
该函数的原型如下:
int regexec (regex_t *compiled, char *string, size_t nmatch, regmatch_t matchptr [], int eflags);
nmatch指明matchptr数组的数目,该数目是compiled->re_nsub+1,也可以让nmatch为0,matchptr为NULL,表示不要保存子表达式。eflags通常为0。
匹配结束后,匹配到的子表达式在串中的偏移保存在regmatch_t结构中,该结构有两个成员:
匹配结束后,匹配到的子表达式在串中的偏移保存在regmatch_t结构中,该结构有两个成员:
- rm_so 子表达式的起始偏移
- rm_eo 子表达式的结束偏移
- regfree
该函数的原型如下:
void regfree(regex_t *preg);
函数regfree()不会返回任何结果,它仅接收一个指向regex_t数据类型的指针,这是之前调用regcomp()函数所得到的编译结果。 如果在程序中针对同一个regex_t结构调用了多次regcomp()函数,POSIX标准并没有规定是否每次都必须调用regfree()函数进行释放,但建议每次调用regcomp()函数对正则表达式进行编译后都调用一次regfree()函数,以尽早释放占用的存储空间。
- regerror
该函数的原型如下
size_t regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size);
参数errcode是来自函数regcomp()或regexec()的错误代码,而参数preg则是由函数regcomp()得到的编译结果,其目的是把格式化消息所必须的上下文提供给regerror()函数。在执行函数regerror()时,将按照参数errbuf_size指明的最大字节数,在 errbuf缓冲区中填入格式化后的错误信息,同时返回错误信息的长度。
POSIX C正则表达式实例:(该代码来自于互联网,很好的示例了POSIX C正则库的几个函数)
/* regexamp_1.c */ #include <stdio.h> #include <sys/types.h> #include <regex.h> /* 取子串的函数 */ static char* substr(const char *str, unsigned start, unsigned end) { unsigned n = end - start; static char stbuf[256]; strncpy(stbuf, str + start, n); stbuf[n] = 0; return stbuf; } int main(int argc, char *argv[]) { char * pattern; int x, z, lno = 0, cflags = 0; char ebuf[128], lbuf[256]; regex_t regex; regmatch_t pm[10]; const size_t nmatch = 10; /* 编译正则表达式*/ pattern = argv[1]; z = regcomp(®ex, pattern, cflags); if (z != 0){ regerror(z, ®ex, ebuf, sizeof(ebuf)); fprintf(stderr, "%s: pattern '%s' \n", ebuf, pattern); return 1; } /* 逐行处理输入的数据 */ while(fgets(lbuf, sizeof(lbuf), stdin)) { ++lno; if ((z = strlen(lbuf)) > 0 && lbuf[z-1] == '\n') lbuf[z - 1] = 0; /* 对每一行应用正则表达式进行匹配 */ z = regexec(®ex, lbuf, nmatch, pm, 0); if (z == REG_NOMATCH) continue; else if (z != 0) { regerror(z, ®ex, ebuf, sizeof(ebuf)); fprintf(stderr, "%s: regcom('%s')\n", ebuf, lbuf); return 2; } /* 输出处理结果 */ for (x = 0; x < nmatch && pm[x].rm_so != -1; ++x) { if (!x) printf("%04d: %s\n", lno, lbuf); printf(" $%d='%s'\n", x, substr(lbuf, pm[x].rm_so, pm[x].rm_eo)); } } /* 释放正则表达式 */ regfree(®ex); return 0; }
编译并运行该代码如下:
$ cc -o regexamp_1 regexamp_1.c $ ./regexamp_1 'regex[a-z]*' < regexamp_1.c 0003: #include $0='regex' 0020: regex_t regex; $0='regex' 0025: z = regcomp(®ex, pattern, cflags); $0='regex' 0027: regerror(z, ®ex, ebuf, sizeof(ebuf)); $0='regex' 0038: z = regexec(®ex, lbuf, nmatch, pm, 0); $0='regexec' 0042: regerror(z, ®ex, ebuf, sizeof(ebuf)); $0='regex' 0055: regfree(®ex); $0='regex'
附regex的man page如下:
REGCOMP(3) Linux Programmer's Manual REGCOMP(3) NAME regcomp, regexec, regerror, regfree - POSIX regex functions SYNOPSIS #include #include int regcomp(regex_t *preg, const char *regex, int cflags); int regexec(const regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags); size_t regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size); void regfree(regex_t *preg); POSIX REGEX COMPILING regcomp() is used to compile a regular expression into a form that is suitable for subsequent regexec() searches. regcomp() is supplied with preg, a pointer to a pattern buffer storage area; regex, a pointer to the null-terminated string and cflags, flags used to determine the type of compilation. All regular expression searching must be done via a compiled pattern buffer, thus regexec() must always be supplied with the address of a regcomp() initialized pattern buffer. cflags may be the bitwise-or of one or more of the following: REG_EXTENDED Use POSIX Extended Regular Expression syntax when interpreting regex. If not set, POSIX Basic Regular Expression syntax is used. REG_ICASE Do not differentiate case. Subsequent regexec() searches using this pattern buffer will be case insensitive. REG_NOSUB Support for substring addressing of matches is not required. The nmatch and pmatch parameters to regexec() are ignored if the pattern buffer supplied was compiled with this flag set. REG_NEWLINE Match-any-character operators don't match a newline. A non-matching list ([^...]) not containing a newline does not match a newline. Match-beginning-of-line operator (^) matches the empty string immediately after a newline, regardless of whether eflags, the execution flags of regexec(), contains REG_NOTBOL. Match-end-of-line operator ($) matches the empty string immedi- ately before a newline, regardless of whether eflags contains REG_NOTEOL. POSIX REGEX MATCHING regexec() is used to match a null-terminated string against the precom- piled pattern buffer, preg. nmatch and pmatch are used to provide information regarding the location of any matches. eflags may be the bitwise-or of one or both of REG_NOTBOL and REG_NOTEOL which cause changes in matching behaviour described below. REG_NOTBOL The match-beginning-of-line operator always fails to match (but see the compilation flag REG_NEWLINE above) This flag may be used when different portions of a string are passed to regexec() and the beginning of the string should not be interpreted as the beginning of the line. REG_NOTEOL The match-end-of-line operator always fails to match (but see the compilation flag REG_NEWLINE above) BYTE OFFSETS Unless REG_NOSUB was set for the compilation of the pattern buffer, it is possible to obtain substring match addressing information. pmatch must be dimensioned to have at least nmatch elements. These are filled in by regexec() with substring match addresses. Any unused structure elements will contain the value -1. The regmatch_t structure which is the type of pmatch is defined in regex.h. typedef struct { regoff_t rm_so; regoff_t rm_eo; } regmatch_t; Each rm_so element that is not -1 indicates the start offset of the next largest substring match within the string. The relative rm_eo element indicates the end offset of the match. POSIX ERROR REPORTING regerror() is used to turn the error codes that can be returned by both regcomp() and regexec() into error message strings. regerror() is passed the error code, errcode, the pattern buffer, preg, a pointer to a character string buffer, errbuf, and the size of the string buffer, errbuf_size. It returns the size of the errbuf required to contain the null-terminated error message string. If both errbuf and errbuf_size are non-zero, errbuf is filled in with the first errbuf_size - 1 characters of the error message and a terminating null. POSIX PATTERN BUFFER FREEING Supplying regfree() with a precompiled pattern buffer, preg will free the memory allocated to the pattern buffer by the compiling process, regcomp(). RETURN VALUE regcomp() returns zero for a successful compilation or an error code for failure. regexec() returns zero for a successful match or REG_NOMATCH for fail- ure. ERRORS The following errors can be returned by regcomp(): REG_BADBR Invalid use of back reference operator. REG_BADPAT Invalid use of pattern operators such as group or list. REG_BADRPT Invalid use of repetition operators such as using `*' as the first character. REG_EBRACE Un-matched brace interval operators. REG_EBRACK Un-matched bracket list operators. REG_ECOLLATE Invalid collating element. REG_ECTYPE Unknown character class name. REG_EEND Non specific error. This is not defined by POSIX.2. REG_EESCAPE Trailing backslash. REG_EPAREN Un-matched parenthesis group operators. REG_ERANGE Invalid use of the range operator, eg. the ending point of the range occurs prior to the starting point. REG_ESIZE Compiled regular expression requires a pattern buffer larger than 64Kb. This is not defined by POSIX.2. REG_ESPACE The regex routines ran out of memory. REG_ESUBREG Invalid back reference to a subexpression. CONFORMING TO POSIX.1-2001. SEE ALSO regex(7), GNU regex manual GNU 1998-05-08 REGCOMP(3)