原文轉自： http://www.devfront.com:8080/?q=node/140
GNU/Linux有兩套庫可用於正則表達式編程：POSIX庫和PCRE庫。前者不需要單獨安裝，一般需求還是能滿足的，速度稍慢些。後者是久負盛名的Perl正則表達式庫，功能強大，匹配速度快，不過可能需要單獨安裝。
我們先用一個例子來介紹如何使用POSIX庫。
＃i nclude <stdio.h>
＃i nclude <sys/types.h>
＃i nclude <regex.h>
char *get_regerror (int errcode, regex_t *compiled)
{
size_t length = regerror (errcode, compiled, NULL, 0);
char *buffer = malloc(length);
if (!buffer) return NULL;
(void) regerror (errcode, compiled, buffer, length);
return buffer;
}
int regtest(const char*pattern, const char* string)
{
regex_t reg;
regmatch_t *subexprs = NULL;
int ret;
int i;
if (0 != (ret=regcomp(?, pattern, REG_EXTENDED))) {
char *buffer = get_regerror(ret, ?);
if (buffer) {
fprintf(stderr, "regcomp:[%d]%s/n", ret, buffer);
free(buffer);
}
return -1;
}
subexprs = malloc((reg.re_nsub+1)*sizeof(regmatch_t));
if (!subexprs) {
fprintf(stderr, "error malloc subexprs/n");
regfree(?);
return -1;
}
if (0 != (ret=regexec(?, string, reg.re_nsub+1, subexprs, 0))) {
char *buffer = get_regerror(ret, ?);
if (buffer) {
fprintf(stderr, "regexec:[%d]%s/n", ret, buffer);
free(buffer);
}
regfree(?);
return -1;
}
for (i = 0; i <= reg.re_nsub; i++) {
printf("[%d]:", i);
if (subexprs[i].rm_so == subexprs[i].rm_eo) {
printf("[EMPTY SUBEXPR]/n");
}
else if (subexprs[i].rm_so == -1 ||
subexprs[i].rm_eo == -1) {
printf("[NO SUBEXPR]/n");
}
else {
fwrite(string+subexprs[i].rm_so, 1,
subexprs[i].rm_eo-subexprs[i].rm_so, stdout);
printf("/n");
}
}
regfree(?);
if (subexprs) free(subexprs);
return 0;
}
int main(int argc, char *argv[])
{
if (argc != 3) {
fprintf(stderr, "Usage: regtest pattern string/n");
return -1;
}
fprintf(stderr, "pattern:%s/n", argv[1]);
fprintf(stderr, "string:%s/n", argv[2]);
return regtest(argv[1], argv[2]);
}
在字符串匹配之前，我們必須先編譯匹配模式，這是通過regcomp實現的。這個函數的原型如下：
參數compiled只有一個成員是我們需要關注的，那就是re_nsub，代表編譯後的子表達式數目，由於我們還需要保存整個匹配到的模式，所以最終匹配的條目數是re_nsub加1。cflags用來修飾匹配模式，可取值如下：
REG_EXTENDED 啓用POSIX正則庫擴展，關於該擴展的詳細信息可參考POSIX規範
REG_ICASE 忽略大小寫
REG_NOSUB 不要存儲子表達式
REG_NEWLINE 把換行符作爲多行的分隔符，這樣'$'可匹配每一行的行尾，'^'匹配每一行的行首，'.'不匹配換行符，[^...]不匹配新行
編譯完模式後我們從內存中分配子表達式存儲空間，然後調用regexec對串進行匹配，該函數原型如下：
nmatch指明matchptr數組的數目，該數目是compiled->re_nsub+1，也可以讓nmatch爲0,matchptr爲NULL，表示不要保存子表達式。eflags通常爲0。
匹配結束後，匹配到的子表達式在串中的偏移保存在regmatch_t結構中，該結構有兩個成員：
rm_so 子表達式的起始偏移
rm_eo 子表達式的結束偏移
這是一個開區間，實際的子表達式在[rm_so,rm_eo)裏。
如果沒有匹配的子表達式，比如"f(o*)"匹配"fum"，實際匹配到的只有"f"，這時rm_so和rm_eo相等，都爲1。如果整個模式在沒有子表示式的情況下也能匹配，這時rm_so和rm_eo爲-1，比如"ba(na)*"匹配"ba"。
POSIX正則庫表述到這裏，下面我們看看PCRE庫。
PCRE庫的功能雖然強大，可是並不難使用。詳細信息可參考http://www.pcre.org/裏的文檔，附錄有一個例子，比較全面地闡述瞭如何調用該庫。
PCRE的子表達式和POSIX類似，不過它還引入了一個命名子表達式的概念。比如模式"(?P(?P(/d/d)?/d/d)-(?P/d/d)-(?P/d/d))"，其中的date, year, month, day是對子表達式的命名。如何根據這些名稱來獲取子表達式，PCRE文檔有詳細的說明。
附錄：pcredemo.c
/*************************************************
* PCRE DEMONSTRATION PROGRAM *
*************************************************/
/* This is a demonstration program to illustrate the most straightforward ways
of calling the PCRE regular expression library from a C program. See the
pcresample documentation for a short discussion.
Compile thuswise:
gcc -Wall pcredemo.c -I/usr/local/include -L/usr/local/lib /
-R/usr/local/lib -lpcre
Replace "/usr/local/include" and "/usr/local/lib" with wherever the include and
library files for PCRE are installed on your system. Only some operating
systems (e.g. Solaris) use the -R option.
*/
＃i nclude <stdio.h>
＃i nclude <string.h>
＃i nclude <pcre.h>
#define OVECCOUNT 30 /* should be a multiple of 3 */
int main(int argc, char **argv)
{
pcre *re;
const char *error;
char *pattern;
char *subject;
unsigned char *name_table;
int erroffset;
int find_all;
int namecount;
int name_entry_size;
int ovector[OVECCOUNT];
int subject_length;
int rc, i;
/**************************************************************************
* First, sort out the command line. There is only one possible option at *
* the moment, "-g" to request repeated matching to find all occurrences, *
* like Perl's /g option. We set the variable find_all to a non-zero value *
* if the -g option is present. Apart from that, there must be exactly two *
* arguments. *
**************************************************************************/
find_all = 0;
for (i = 1; i < argc; i++) {
if (strcmp(argv[i], "-g") == 0) find_all = 1;
else break;
}
/* After the options, we require exactly two arguments, which are the pattern,
and the subject string. */
if (argc - i != 2) {
printf("Two arguments required: a regex and a subject string/n");
return 1;
}
pattern = argv[i];
subject = argv[i+1];
subject_length = (int)strlen(subject);
/*************************************************************************
* Now we are going to compile the regular expression pattern, and handle *
* and errors that are detected. *
*************************************************************************/
re = pcre_compile(
pattern, /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */
/* Compilation failed: print the error message and exit */
if (re == NULL) {
printf("PCRE compilation failed at offset %d: %s/n", erroffset, error);
return 1;
}
/*************************************************************************
* If the compilation succeeded, we call PCRE again, in order to do a *
* pattern match against the subject string. This does just ONE match. If *
* further matching is needed, it will be done below. *
*************************************************************************/
rc = pcre_exec(
re, /* the compiled pattern */
NULL, /* no extra data - we didn't study the pattern */
subject, /* the subject string */
subject_length, /* the length of the subject */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* output vector for substring information */
OVECCOUNT); /* number of elements in the output vector */
/* Matching failed: handle error cases */
if (rc < 0) {
switch(rc) {
case PCRE_ERROR_NOMATCH: printf("No match/n"); break;
/*
Handle other special cases if you like
*/
default: printf("Matching error %d/n", rc); break;
}
free(re); /* Release memory used for the compiled pattern */
return 1;
}
/* Match succeded */
printf("/nMatch succeeded at offset %d/n", ovector[0]);
/*************************************************************************
* We have found the first match within the subject string. If the output *
* vector wasn't big enough, set its size to the maximum. Then output any *
* substrings that were captured. *
*************************************************************************/
/* The output vector wasn't big enough */
if (rc == 0) {
rc = OVECCOUNT/3;
printf("ovector only has room for %d captured substrings/n", rc - 1);
}
/* Show substrings stored in the output vector by number. Obviously, in a real
application you might want to do things other than print them. */
for (i = 0; i < rc; i++) {
char *substring_start = subject + ovector[2*i];
int substring_length = ovector[2*i+1] - ovector[2*i];
printf("%2d: %.*s/n", i, substring_length, substring_start);
}
/**************************************************************************
* That concludes the basic part of this demonstration program. We have *
* compiled a pattern, and performed a single match. The code that follows *
* first shows how to access named substrings, and then how to code for *
* repeated matches on the same subject. *
**************************************************************************/
/* See if there are any named substrings, and if so, show them by name. First
we have to extract the count of named parentheses from the pattern. */
(void)pcre_fullinfo(
re, /* the compiled pattern */
NULL, /* no extra data - we didn't study the pattern */
PCRE_INFO_NAMECOUNT, /* number of named substrings */
&namecount); /* where to put the answer */
if (namecount <= 0) printf("No named substrings/n"); else
{
unsigned char *tabptr;
printf("Named substrings/n");
/* Before we can access the substrings, we must extract the table for
translating names to numbers, and the size of each entry in the table. */
(void)pcre_fullinfo(
re, /* the compiled pattern */
NULL, /* no extra data - we didn't study the pattern */
PCRE_INFO_NAMETABLE, /* address of the table */
&name_table); /* where to put the answer */
(void)pcre_fullinfo(
re, /* the compiled pattern */
NULL, /* no extra data - we didn't study the pattern */
PCRE_INFO_NAMEENTRYSIZE, /* size of each entry in the table */
&name_entry_size); /* where to put the answer */
/* Now we can scan the table and, for each entry, print the number, the name,
and the substring itself. */
tabptr = name_table;
for (i = 0; i < namecount; i++) {
int n = (tabptr[0] << 8) | tabptr[1];
printf("(%d) %*s: %.*s/n", n, name_entry_size - 3, tabptr + 2,
ovector[2*n+1] - ovector[2*n], subject + ovector[2*n]);
tabptr += name_entry_size;
}
}
/*************************************************************************
* If the "-g" option was given on the command line, we want to continue *
* to search for additional matches in the subject string, in a similar *
* way to the /g option in Perl. This turns out to be trickier than you *
* might think because of the possibility of matching an empty string. *
* What happens is as follows: *
* *
* If the previous match was NOT for an empty string, we can just start *
* the next match at the end of the previous one. *
* *
* If the previous match WAS for an empty string, we can't do that, as it *
* would lead to an infinite loop. Instead, a special call of pcre_exec() *
* is made with the PCRE_NOTEMPTY and PCRE_ANCHORED flags set. The first *
* of these tells PCRE that an empty string is not a valid match; other *
* possibilities must be tried. The second flag restricts PCRE to one *
* match attempt at the initial string position. If this match succeeds, *
* an alternative to the empty string match has been found, and we can *
* proceed round the loop. *
*************************************************************************/
if (!find_all) {
free(re); /* Release the memory used for the compiled pattern */
return 0; /* Finish unless -g was given */
}
/* Loop for second and subsequent matches */
for (;;) {
int options = 0; /* Normally no options */
int start_offset = ovector[1]; /* Start at end of previous match */
/* If the previous match was for an empty string, we are finished if we are
at the end of the subject. Otherwise, arrange to run another match at the
same point to see if a non-empty match can be found. */
if (ovector[0] == ovector[1]) {
if (ovector[0] == subject_length) break;
options = PCRE_NOTEMPTY | PCRE_ANCHORED;
}
/* Run the next matching operation */
rc = pcre_exec(
re, /* the compiled pattern */
NULL, /* no extra data - we didn't study the pattern */
subject, /* the subject string */
subject_length, /* the length of the subject */
start_offset, /* starting offset in the subject */
options, /* options */
ovector, /* output vector for substring information */
OVECCOUNT); /* number of elements in the output vector */
/* This time, a result of NOMATCH isn't an error. If the value in "options"
is zero, it just means we have found all possible matches, so the loop ends.
Otherwise, it means we have failed to find a non-empty-string match at a
point where there was a previous empty-string match. In this case, we do what
Perl does: advance the matching position by one, and continue. We do this by
setting the "end of previous match" offset, because that is picked up at the
top of the loop as the point at which to start again. */
if (rc == PCRE_ERROR_NOMATCH) {
if (options == 0) break;
ovector[1] = start_offset + 1;
continue; /* Go round the loop again */
}
/* Other matching errors are not recoverable. */
if (rc < 0) {
printf("Matching error %d/n", rc);
free(re); /* Release memory used for the compiled pattern */
return 1;
}
/* Match succeded */
printf("/nMatch succeeded again at offset %d/n", ovector[0]);
/* The match succeeded, but the output vector wasn't big enough. */
if (rc == 0) {
rc = OVECCOUNT/3;
printf("ovector only has room for %d captured substrings/n", rc - 1);
}
/* As before, show substrings stored in the output vector by number, and then
also any named substrings. */
for (i = 0; i < rc; i++) {
char *substring_start = subject + ovector[2*i];
int substring_length = ovector[2*i+1] - ovector[2*i];
printf("%2d: %.*s/n", i, substring_length, substring_start);
}
if (namecount <= 0) printf("No named substrings/n"); else
{
unsigned char *tabptr = name_table;
printf("Named substrings/n");
for (i = 0; i < namecount; i++)
{
int n = (tabptr[0] << 8) | tabptr[1];
printf("(%d) %*s: %.*s/n", n, name_entry_size - 3, tabptr + 2,
ovector[2*n+1] - ovector[2*n], subject + ovector[2*n]);
tabptr += name_entry_size;
}
}
} /* End of loop to find second and subsequent matches */
printf("/n");
free(re); /* Release memory used for the compiled pattern */
return 0;
}
/* End of pcredemo.c */