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Date: Thu, 11 Apr 2002 21:26:09 +0300
From: "Eli Zaretskii" <eliz AT is DOT elta DOT co DOT il>
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To: "Yong Kwang Goh" <gohyongkwang AT hotmail DOT com>
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In-reply-to: <a94dfn$a91$1@violet.singnet.com.sg> (gohyongkwang@hotmail.com)
Subject: Re: Allocate memory for pmatch[] prior to calling regexec()?
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> From: "Goh, Yong Kwang" <gohyongkwang AT hotmail DOT com>
> Newsgroups: comp.os.msdos.djgpp
> Date: Thu, 11 Apr 2002 16:51:10 +0800
> 
> int regexec(const regex_t *preg, const char *string, size_t nmatch,
> regmatch_t pmatch[], int eflags);
> 
> So prior to calling regexec(), do we have to allocate memory for the
> pmatch[] array of regmatch_t structures or it'll be dynamically allocated by
> regexec() itself as it finds sub-expressions matches, conforming to the
> compiled regex, in the string?

You have to allocate it.

> If we have to allocate memory for pmatch[], how do we know how many
> sub-expressions there are in the regex, assuming the regex is only known
> when the program is run

The number of subexpressions is returned by regcomp in the
preg->re_nsub member (see the docs of regcomp, which explains this).

> Also, one thing that I cannot understand is that that size_t nmatch is
> supposed to hold the number of matches (sub-expressions) it could find in
> the string.

This argument is input to regexec, not output from it.  So it's
passed by value and is not changed by regexec.

> In another word, the size of pmatch[] is already fixed and memory
> has already been allocated for pmatch[] prior to calling regexec.

That's correct.