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EugenMayerTalk

Regular expressions

Regular expressions

I'm not quite sure what you are looking for, but I believe there are two general approaches that can be used:

If it's just one character you need to check for, as I think you are trying to do in this case (check for /), then you can explicitly have a check for a leading character that is not / or any other special character. (Note if you also needed to match zero characters, this would not work, but in this case I believe it is acceptable to not insert $docroot into an empty value for the src attribute.)
You can use negative lookahead: (?!pattern_to_not_match)

So taking your simple example of trying to replace <img src="somefile.png"> with <img src="$docroot/somefile.png"> but not touch <img src="/somefile.png">, you could use a regexp like either of the following:

  $newString =~ s{
     (<[iI][mM][gG]
        \s+
        (?:[^>]+\s+)*
        [sS][rR][cC] \s*=\s*)   # $1
        (['"])                  # $2
        ([^/\2]
         [^\2]*)                # $3
        (\2
          [^>]*>
        )                       # $4
  }{$1$2$docroot$3$4}gx;

  $newString =~ s{
     (<[iI][mM][gG]
        \s+
        (?:[^>]+\s+)*
        [sS][rR][cC] \s*=\s*)   # $1
        (['"])                  # $2
        ((?!/)
         [^\2]+)                # $3
        (\2
          [^>]*>
        )                       # $4
  }{$1$2$docroot$3$4}gx;

-- IsaacLin - 03 Dec 2008 - 22:54

I used the regexps above as examples in a tutorial I am creating, and realized there are some cases they won't cover, such as src= appearing within the value of another attribute. Here is a more complete solution:

Edited on 20 Feb 2009: changed how attribute values are matched, as backreferences within a character class does not seem to work. The example below only works with src attributes that have a double-quoted value; either repeat the regexp for single-quoted values (and possibly values without quotes), or add the /e modifier and have a small fragment of code to extract the exact value and then prepend $docroot.

my $reSqString = qr{
  \'
  [^\']*
  \'
}x;

my $reDqString = qr{
  \"
  [^\"]*
  \"
}x;

my $reAttrValue = qr{
  (?: $reSqString | $reDqString | [^\'\"\s]+ )
}x;

$newString =~ s{
    (<[iI][mM][gG]             # $1 start
       \s+
       (?: \w+ \s*=\s* $reAttrValue \s+ )*
       [sS][rR][cC] \s*=\s*)   # $1 end
      (?:\"([^\"]+)\")  # $2
      ( (?: \s+ \w+ \s*=\s* $reAttrValue )* #$3
        \s*/?>
      )                       # $3 end
}{$1"$docroot$2"$3}gx;

It assumes that attribute names always match \w+, and that the values are properly enclosed in quotes.

-- IsaacLin - 04 Feb 2009