Re: Look-Arounds in Regexes are Hard

[papidave]

++moritz for caring about beginners.

I don't consider myself to be a newby any more, but even so, look-wherever expressions make me a bit crosseyed, and it got worse when I realized that the original question implied a fixed start and end to the pattern. When that happens, a line like

   Functions <code>abc()</code> and <code>foo()</code>

should match the first code block, but not the second.

If we want to use lookahead to match <code> blocks that don't contain foo, we might use something like

   m#<code>((?:(?!foo).)*)</code>#

but you'd be wise to insert a ton of comments to clarify things*. OTOH, I find it far more readable to use something like the following:

#!/usr/bin/perl -w

use strict;

LINE: while ( <DATA> ) 
{
   MATCH: while ( m#<code>(.*?)</code>#gi ) 
   {
      print "$1\n" if ( $1 !~ /foo/ );
   }
}

__DATA__
This has no match at all, and is skipped.
This has foo, but is skipped with no code block.
<code>This is acceptable</code>
so is <code></code> an embedded block, and the next.
<code></code>
<code>is foo rejected</code>
<code>foo</code>
we will skip <code> with no terminator.
and, we will also skip <code> with foo, but no terminator.
foo that preceeds <code> just </code> simple text is accepted.
here is <code> another </code> example.
but <code> Perl </code> will accept foo outside the block.
The foo can be in advance, as every <code> hacker </code> knows.
Doubled matches like <code></code> foo <code></code> are evil.
This should <code> skip the second loop</code><code> foo </code>.
This <code> foo </code> should  <code> skip the first loop </code>.

This gives us the desired multiple-block behaviors, while providing us with the non-foo data (in $1) for each instance, so we can do something more complicated with it if desired. In addition, the nested loop was surprisingly about 10% faster than the look-ahead code when I compared it over 100,000 iterations using Perl v5.10.0.

*Unless you're in the UK; in that case, use a tonne of comments instead. :)

Note: lost a slash in code cut/paste business - repaired.