I am working on a large script to analyze code. I am trying to find key words in code, but not if they are in commented lines. Thus, if I am looking for foo, I want to match

maybe some stuff foo maybe some more stuff;
[download]

but not

// maybe some stuff foo maybe some more stuff;
[download]

This looks like an appropriate place to use a negative lookbehind assertion. I have not found the right form yet. Experimenting, I have tried

$a="abcdefghijklm";
1 print "match" if ( $a =~ /(?<!cde).*?jkl/ );    # match because a do
+es not match cde, then bcdefghi follows, then jkl matches?
2 print "match" if ( $a =~ /(?<=cde).*?jkl/ );    # match because cde 
+matches then fgh follws, then jkl matches

$a="xyzfghijklm";
1 print "match" if ( $a =~ /(?<!cde).*?jkl/ );    # match because cde 
+is not before jkl, then jkl matches
2 print "match" if ( $a =~ /(?<=cde).*?jkl/ );    # no match because c
+de is not before jkl
[download]

I am looking for a regex that will not match abcdefghijklm because cde precedes jkl, but would match xyzfghijkl. I would have thought that something like the first one above would be what I wanted.

If I wanted it to not match when jkl must not be preceded by only c, the answer is easy: ^[^c]*jkl. But I have multiple character cde things I must exclude.

I am guessing that what I am looking for will have a (?<!cde) and then a jkl in it.

One trick I used several years ago, seems to work here. Since you cannot do variable length lookbehinds, I can reverse the strings and then do variable length lookaheads. i.e.

$a = "mlkjihgfedcba";
print "match" if ( $a =~ /lkj(?!.*?edc/ );    # no match because edc D
+OES follow lkj. This is what is desired. (It does not match in the un
+reversed when cde precedes jkl)

$a = "mlkjihgfzyx";
print "match" if ( $a =~ /lkj(?!.*?edc/ );    # match because edc does
+ not follow lkj. Again, this is what I want. ( It does match jkl in t
+he unreversed when not preceded by cde)
[download]

Is there another way?

I am guessing this is one of those areas where regexes come up short. I just learned about (?{ }) and (??{ }), but don't fully understand them yet. Where is a good place to learn advanced regex aside from perlre and Friedl?

I am limited to perl 5.6, if that makes a difference. I cannot use CPAN or any other libraries.

If this question seems disjoint, it is because my brain stores in hashes instead of sorted arrays.

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