The regex  qr{(?<!foo)\s+bar} will not match
          'foo    bar'
              ^^
              ||
   no match --++-- can match here
at the first position because it's preceded by 'foo', but at the second position it's not preceded by 'foo' but by a whitespace character, so it can match!

Update: If you have Perl version 5.10+, here's one way to simulate a variable-width negative look-behind: 

c:\@Work\Perl>perl -wMstrict -le
"use 5.010;
 ;;
 my $s = 'foo 1 foo  2 foo   3 4 x 567 foo foo 8 foo9';
 my @matches = $s =~ m{ (?> foo \s+ \d+ (*SKIP)(*FAIL))? \s+ \d+}xmsg;
 printf qq{'$_'  } for @matches;
"
' 4'  ' 567'

[download]
See Special Backtracking Control Verbs in perlre for  (*SKIP) (*FAIL) and friends.
See also Look Behind issues and Another Look behind for further recent discussion of the variable-width negative look-behind issue. (Update: Variable-width positive look-behind is nicely handled by the  \K operator.)

Give a man a fish:  <%-(-(-(-<

In reply to Re: look behind with multiple whitespace characters by AnomalousMonk
in thread look behind with multiple whitespace characters by RK

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