Before you can actually match a regular expression, you must compile it. This is not true compilation—it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to “execute” the pattern fast. (See Matching POSIX Regexps, for how to use the compiled regular expression for matching.)
There is a special data type for compiled regular expressions:
This type of object holds a compiled regular expression. It is actually a structure. It has just one field that your programs should look at:
- This field holds the number of parenthetical subexpressions in the regular expression that was compiled.
There are several other fields, but we don't describe them here, because only the functions in the library should use them.
After you create a
regex_t object, you can compile a regular
expression into it by calling
regcomp“compiles” a regular expression into a data structure that you can use with
regexecto match against a string. The compiled regular expression format is designed for efficient matching.
regcompstores it into
It's up to you to allocate an object of type
regex_tand pass its address to
The argument cflags lets you specify various options that control the syntax and semantics of regular expressions. See Flags for POSIX Regexps.
If you use the flag
regcompomits from the compiled regular expression the information necessary to record how subexpressions actually match. In this case, you might as well pass
0for the matchptr and nmatch arguments when you call
If you don't use
REG_NOSUB, then the compiled regular expression does have the capacity to record how subexpressions match. Also,
regcomptells you how many subexpressions pattern has, by storing the number in compiled
->re_nsub. You can use that value to decide how long an array to allocate to hold information about subexpression matches.
0if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below). You can use
regerrorto produce an error message string describing the reason for a nonzero value; see Regexp Cleanup.
Here are the possible nonzero values that
regcomp can return:
regcompran out of memory.