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Re: Perl & regex help

by choroba (Abbot)
on Jan 30, 2013 at 22:30 UTC ( #1016166=note: print w/ replies, xml ) Need Help??


in reply to Perl & regex help

To avoid &, use a negative look-ahead:

/&(?!amp;)/
i.e. the & not followed by amp;.
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Re^2: Perl & regex help
by Kenosis (Priest) on Jan 30, 2013 at 22:52 UTC

    Ah, yes. Perhaps a more general form for the html entities, e.g.,:

    /&(?!.+?;)/

    Update: I have no idea what I was 'thinking' here. Excellent and appreciated corrections to this below...

      It fails on a string like this: "I saw a dog & a cat;".

      A safer solution, would be: /&(?!#?[a-zA-Z0-9]+;)/

        Excellent catch! Perhaps just the following: /&(?!\S+?;)/

        .

      That won't just exclude HTML entities from being matched, it will exclude any & character that is in the same line as a semicolon somewhere to the right of it, because .+? also matches whitespace.

      Instead, you should match for HTML/XML entities specifically. There are three forms that they can take, and the corresponding regexes for matching them would be:

        /&#[0-9]+;/ - character referenced by decimal number

        /&#x[0-9a-f]+;/i - character referenced by hexadecimal number

        /&[a-z]+;/i - character referenced by name

      Putting it together, you get this regex for matching an HTML entity:
        /&(?:#(?:[0-9]+|x[0-9a-f]+)|[a-z]+);/i

      Although that's kinda messy and pedantic, and you can probably get away with using this simplified version:
        /&#?[0-9a-z]+;/i
      (Unlike the more pedantic version, it would match some false positives such as &#amp; or &1a2b3c;, but what are the chances such constructs will appear in the input document?)

      To do what the OP requested, wrap everything after the & in a negative look-ahead bracket like choroba suggested:

      # 10 20 30 40 50 # ---------'---------'---------'---------'---------'---- my $str = "& ... & ... & ... &no_entity; ... & ... ;"; while ($str =~ /&(?!#?[0-9a-z]+;)/gi) { print "Found ampersand at position ".pos($str)."\n"; }

      Output:

      Found ampersand at position 32 Found ampersand at position 48

      (i.e. it only matches the last two & characters in $str)

        I think your regex is the best, as it clearly aligns with the ISO specs. Nice work.

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