From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <224a39ebca9aaa0370eb804cd59e6aac@plan9.jp>
To: 9fans@cse.psu.edu
Subject: Composition of regexps (Was re: [9fans] regular expressions in plan9
	different from the ones in unix?)
From: Joel Salomon <JoelCSalomon@Gmail.com>
Date: Fri, 23 Feb 2007 01:27:56 -0500
In-Reply-To: <ee9e417a0702221548l2caf1a85m276c7b34e2e55b65@mail.gmail.com>
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On 2/22/07, Russ Cox <rsc@swtch.com> wrote:
> The Plan 9 regexp library matches the old Unix egrep command.
> Any regexp you'd try under Plan 9 should work with new egreps,
> though not vice versa -- new egreps tend to have newfangled
> additions like [:upper:] and \w and {4,6} for repetition.

This came up as I was implementing my C lexer for the compilers class
I'm taking.  How hard would it be to allow access to regcomp(2)'s
internals, so I could build up a regexp part-by part a la lex?

For example, to recognize C99 hexadecimal floating-point constants, I
wrote a second program that builds up the regexp piece-by-piece using
smprint(2), then compiling the whole thing:

	char	*decdig =3D "([0-9])",
		*hexdig =3D "([0-9A-Fa-f])",
		*sign =3D "([+\\-])",
		*dot =3D "(\\.)",
		*dseq, *dexp, *dfrac, *decflt,
		*hseq, *bexp, *hfrac, *hexflt;
	dseq =3D smprint("(%s+)", decdig);
	dexp =3D smprint("([Ee]%s?%s)", sign, dseq);
	dfrac =3D smprint("((%s?%s%s)|(%s%s))", dseq, dot, dseq, dseq, dot);
	decflt =3D smprint("(%s%s?)|(%s%s)", dfrac, dexp, dseq, dexp);
	regcomp(decflt);	// make sure it compiles
	print("decfloat: %s\n", decflt);
=09
	hseq =3D smprint("(%s+)", hexdig);
	bexp =3D smprint("([Pp]%s?%s)", sign, dseq);
	hfrac =3D smprint("((%s?%s%s)|(%s%s))", hseq, dot, hseq, hseq, dot);
	hexflt =3D smprint("0[Xx](%s|%s)%s", hfrac, hseq, bexp);
	regcomp(hexflt);	// make sure it compiles
	print("hexfloat: %s\n", hexflt);

I know that regcomp builds up the Reprog by combining subprograms with
catenation and alternation &c., but I=E2=80=99d be loath to try tinkering
there directly without a much better understanding of the algorithm.
I=E2=80=99ve glanced through the documents at swtch.com/?????  and the re=
gcomp
source code, just haven=E2=80=99t had the time for an in-depth study.

Would such a project be a worthwhile spent of time?  (Might it develop
into the asteroid to kill the dinosaur waiting for it?)

--Joel