From: Thomas Hurst <tom.hurst@...>
Date: 2002-04-11T03:12:15+09:00
Subject: Re: regex: how to negate sequences?

* Tobias Reif (tobiasreif@pinkjuice.com) wrote:

> Can you explain the ?= syntax, or point to it's doc?

perldoc perlre

  (?=regexp)
  A zero-width positive lookahead assertion. For example, /\w+(?=\t)/
  matches a word followed by a tab, without including the tab in $&.

There's also:

  (?!regexp)
  A zero-width negative lookahead assertion. For example /foo(?!bar)/
  matches any occurrence of ``foo'' that isn't followed by ``bar''.

But note this isn't the same as negation; it's asserting that the area
ahead of the match does or does not contain something.

Another useful one is:

  (?:regexp)
  This groups things like ``()'' but doesn't make backrefences like ``()''
  does.

So if you want to match a string but not save the value:

/foo (?:bar|fee)(.*)/

Will result in $1 pointing to the second set of ()'s.

You can also embed pattern modifiers in there:

irb(main):001:0> 'CASE INSENSITIVE' =~ /[A-Z]+ (?i)[a-z]+/ ; p $&
"CASE INSENSITIVE"

-- 
Thomas 'Freaky' Hurst  -  freaky@aagh.net  -  http://www.aagh.net/
-
Good salesmen and good repairmen will never go hungry.
		-- R.E. Schenk