On 12.03.2007 14:22, Greg Hurrell wrote:
> Given the following regular expression:
> 
> /([^*#]+|#(?!#|\*)|\*(?!#))+/
> 
> I wanted to make it more readable by inserting some comments, so I
> tried adding the "x" option and it no longer compiled:
> 
> /([^*#]+|#(?!#|\*)|\*(?!#))+/x
> 
> If you try it in irb you'll see a message similar to this:
> 
> SyntaxError: compile error
> (irb):31: unmatched (: /([^*#]+|#(?!#|\*)|\*(?!#))+/
> 
> To get this to compile I had to add additional backslashes to escape
> the '#' character in the negative lookahead subexpressions:
> 
> /([^*#]+|#(?!\#|\*)|\*(?!\#))+/x
> 
> The '#' character normally matches itself in a regular expression.
> With the "x" option I expect it to have a special meaning (indicating
> a comment) but in one special position (immediately after the opening
> brace and question mark):
> 
> (?# comment )
> 
> Is this a bug in the regular expression engine, undocumented or am I
> missing something?
> 
> No big deal, the thing is compiling, but I'd like to understand this a
> bit better.

A "#" all by itself introduces a line comment as in normal code.  So 
everything after it is treated as a comment:

15:43:52 [~]: ruby -e 'r=/foo#bar
baz/x; p r; p r =~ "foobaz"'
/foo#bar
baz/x
0

The regexp matches "foobaz". /x allows line comments - (?#...) works always:

15:46:08 [~]: ruby -e 'r=/foo(?#bar)baz/; p r; p r =~ "foobaz"'
/foo(?#bar)baz/
0

Kind regards

	robert