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(libc.info)Regexp Subexpressions

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Match Results with Subexpressions
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   When `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 subexpression.

 - Data Type: regmatch_t
     This is the data type of the MATCHARRAY array that you pass to
     `regexec'.  It contains two structure fields, as follows:

    `rm_so'
          The offset in STRING of the beginning of a substring.  Add
          this value to STRING to get the address of that part.

    `rm_eo'
          The offset in STRING of the end of the substring.

 - Data Type: regoff_t
     `regoff_t' is an alias for another signed integer type.  The
     fields of `regmatch_t' have type `regoff_t'.

   The `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 `regexec' how 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 `0' for NMATCH, or
use the flag `REG_NOSUB' when you compile the pattern with `regcomp'.