2004/11/15 regex

Some material is very old and may be incorrect today

© November 2004 Tony Lawrence

Regular Expression. The type of pattern matching you do with sed or grep or Perl uses regexes.

There are whole books on regular expressions. I imagine most of us have some degree of familiarity, but this can be a very complex subject. Larry Wall (originator of Perl) hit the nail on the head when he said

Regexes aren't strings, they're programs. Or, rather, they're
strings only in the sense that any piece of program is a string
 

So, in the Perl world, regex's will undergo a major transformation with Perl 6. Larry (a bit of a religious nut) describes the changes in Perl 6 as an Apocalypse. Don't get too worried: Larry promises

It is my fond hope that those who are fond of Perl 5 will be fonder
still of Perl 6
 

But back to regexes. You'll find the gory details of Larry's thoughts on regexes, what's wrong, what needs to be changed, and more, at http://www.perl.com/pub/a/2002/06/04/apo5.html. If you use Perl, you'll want to read that. Again, Larry warns you:

Given all this, I need to warn you that this Apocalypse is going
to be somewhat radical. We'll be proposing changes to certain
"sacred" features of regex culture, and this is guaranteed to result
in future shock for some of our more conservative citizens. Do not
be alarmed. We will provide ways for you to continue programming
in old-fashioned regular expressions if you desire. But I hope that
once you've thought about it a little and worked through some
examples, you'll like most of the changes we're proposing here.
 

If Larry's expository style isn't quite your cup of tea (I enjoy reading it, but not everyone agrees), you might try Perl 6 grammars and regular expressions for another view of the changes coming.

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