Question

I’m trying to write a regular expression that surrounds ‘http’ URLs with angle brackets, except for lines beginning with two slashes. The best I’ve come up with is:

s#^(?!//)(.*?)(http://[^\s]+)#$1<$2>#gm;

This works great for these two:

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Input: http://a.com

Output: <http://a.com>

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Input: //http://a.com

Output: //http://a.com

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However, it fails here:

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Input: http://a.com http://b.com

Actual Output: <http://a.com> http://b.com

Desired Output: <http://a.com> <http://b.com>

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Why doesn’t my regular expression keep matching? Am I using /g wrong?

Answer 1

rewriting it a little…with my suggestions and using the whitespace modifier so it’s actually readable. 🙂

s{     (?:^|\G)     # start of the last match, so you never backtrack and don't capture.     (?!//)       # a section without //     (.*?)        # followed by anything     (         http://  # with http://         [^\s]+   # and non-spaces - you could also use \S     )  }  {$1<$2>}xmg; 

Trying this in perl, we get:

sub test {     my ($str, $expect) = @_;     my $mod = $str;     $mod =~ s{             (?:^|\G)       # start of the last match, so you never backtrack.             (?!//)       # a section without //             (.*?)        # followed by anything             (                 http://  # with http://                 [^\s]+   # and non-spaces - you could also use \S             )           }           {$1<$2>}xmg;     print 'Expecting '$expect' got '$mod' - ';     print $mod eq $expect ? 'passed\n' : 'failed\n'; }  test('http://foo.com',    '<http://foo.com>'); test('// http://foo.com', '// http://foo.com'); test('foo\nhttp://a.com','foo\n<http://a.com>');  # output is  # Expecting '<http://foo.com>' got '<http://foo.com>' - passed # Expecting '// http://foo.com' got '// http://foo.com' - passed # Expecting 'foo # <http://a.com>' got 'foo # <http://a.com>' - passed 

Edit: Couple of changes: Added the ‘m’ modifier to make sure that it matches from the start of a line, and change \G to (^|\G) to make sure it starts looking at the start of the line too.