Boolean operators in PERL regexp?

lenrobert has asked for the wisdom of the Perl Monks concerning the following question:

I am aware of OR ( | ), but is there logical NOT in the PERL regex syntax?

The task would be the following: to extract the relative links (i.e. the href property of the "a" element) from an HTML file, even if it is not enclosed in quotation marks. This means I don't want to retrieve hyperlinks beginning with /, or # or javascript:

I would express the following string, and capture (extract) the content of the second parenthesis.

( <a href=" OR <a href=) THEN NOT(/ OR # OR javascript: OR \s OR " ) THEN ( \s OR " )

The best regexp I could do is this, but it does not handle the case of / # javascript: etc.

/(<a href="|<a href=)([^"]*?)(\s|")/gi)

Does anyone know the answer, and help me? Thanks in advance,

Robert

Comment on Boolean operators in PERL regexp? Download Code

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Re: Boolean operators in PERL regexp? by friedo (Prior) on Feb 24, 2005 at 18:45 UTC
Parsing HTML with regular expressions can only end in tears. I would suggest checking out HTML::Parser. I think the closest thing to what you're looking for is the zero-width negative lookahead assertion. For example, /foo(?!bar)/ matches "foo" followed by anything that isn't "bar". See perlre for more information; negative lookaheads are difficult to master and can become quite complex.	[reply]
Re: Boolean operators in PERL regexp? by Enlil (Parson) on Feb 24, 2005 at 18:55 UTC
`m#<a href="?(?!/\|#\|javascript)([^">]*)#` or something near that. Note that there are much better tools for parsing HTML than regular expressions. Try HTML::TokeParser or HTML::Parser, and save yourself a headache. -enlil	[reply] [d/l]
Re: Boolean operators in PERL regexp? by ikegami (Patriarch) on Feb 24, 2005 at 19:02 UTC
I recommend against using these (a parser would work better), but here's a snippet from my scratchpad: `/^(?:(?!$re).)$/ # NOT re /$re1\|$re2/ # re1 OR re2 /^(?=.$re1)(?=.$re2)/ # re1 AND re2` [download] NOT must be anchored on both ends, but it doesn't have to be with `^` and `$`. AND doesn't have to be anchored, but if the start is anchored (with `^` or by some other means), it should speed up the case where there is no match. The `.` in AND may need to be replaced so it doesn't look too far ahead.	[reply] [d/l] [select]
Re^2: Boolean operators in PERL regexp? by lenrobert (Initiate) on Feb 25, 2005 at 16:25 UTC
Friedo, Enlinl, Ikegami, thank you for your responses. Yes, my problem might be easily solved with an HTML parser, but beyond my question in my opinion a logical NOT would be essential for regexps. Lookahead and lookbehind assertions are very good, the only fatal problem with them, that they are non-capturing, i.e. I cannot extract non-matching. Maybe there is a reason for this, I don't know. Robert	[reply]
Re^3: Boolean operators in PERL regexp? by ikegami (Patriarch) on Feb 25, 2005 at 16:30 UTC
Whatcha talking about? `NOT /XYZ/ vvvvvvvvvvvvv >perl -e "$_ = 'ABCDEFGHIJKL'; print(/ABC((?:(?!XYZ).)*)JKL/);" DEFGHI` [download]	[reply] [d/l]
Re^4: Boolean operators in PERL regexp? by lenrobert (Initiate) on Feb 25, 2005 at 17:41 UTC

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