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regexec matches parenthetical subexpressions of
pattern, it records which parts of string they match. It
returns that information by storing the offsets into an array whose
elements are structures of type
regmatch_t. The first element of
the array (index
0) records the part of the string that matched
the entire regular expression. Each other element of the array records
the beginning and end of the part that matched a single parenthetical
This is the data type of the matchptr array that you pass to
regexec. It contains two structure fields, as follows:
The offset in string of the beginning of a substring. Add this value to string to get the address of that part.
The offset in string of the end of the substring.
regoff_t is an alias for another signed integer type.
The fields of
regmatch_t have type
regmatch_t elements correspond to subexpressions
positionally; the first element (index
1) records where the first
subexpression matched, the second element records the second
subexpression, and so on. The order of the subexpressions is the order
in which they begin.
When you call
regexec, you specify how long the matchptr
array is, with the nmatch argument. This tells
many elements to store. If the actual regular expression has more than
nmatch subexpressions, then you won’t get offset information about
the rest of them. But this doesn’t alter whether the pattern matches a
particular string or not.
If you don’t want
regexec to return any information about where
the subexpressions matched, you can either supply
nmatch, or use the flag
REG_NOSUB when you compile the