Re: [9fans] non greedy regular expressions

[Aharon Robbins <arnold@skeeve.com>]

You are not missing anything.

Subexpression matching means when you have an expression like

	q(a+b)(c*d)z

that you can get access to the exact text matched by the two
parenthesized subexpressions.

You asked about non-greedy regular expressions which were first
popularized by perl.

IIRC the Plan 9 regex library does not provide this at all; Bell Labs
code never did non-greedy regexp matching.

Rob and/or Russ can correct me if I'm wrong.

FWIW, tools from the GNU world also do not support non-greedy
matching, nor are such expressions part of POSIX.

Hope this helps,

Arnold

> > russ has a great writeup on this.
> > http://swtch.com/~rsc/regexp/
> > i think it covers all your questions.
> >
> > - erik
>
> I read trough some of that already yesterday. Anyway, am still
> puzzled. In the text of
>
> Regular Expression Matching Can Be Simple And Fast
> (but is slow in Java, Perl, PHP, Python, Ruby, ...)
>
> R. Cox writes:
> ---
> While writing the text editor sam [6] in the early 1980s, Rob Pike
> wrote a new regular expression implementation, which Dave Presotto
> extracted into a library that appeared in the Eighth Edition. Pike's
> implementation incorporated submatch tracking into an efficient NFA
> simulation but, like the rest of the Eighth Edition source, was not
> widely distributed.
> ...
> Pike's regular expression implementation, extended to support Unicode,
> was made freely available with sam in late 1992, but the particularly
> efficient regular expression search algorithm went unnoticed. The code
> is now available in many forms: as part of sam, as Plan 9's regular
> expression library, or packaged separately for Unix.
> ---
>
> But any manual page (regexp(6), that of sam)  keeps completely silent
> about eg. any submatch tracking.
> So what's wrong? Can anybody clarify the situation for me or do I
> really have to read the codes?
>
> Ruda
>