Question

I want to know how this ambiguous pattern is solved in perl(more generally everything that use libpcre):

/(\r\n|\r|\n)/

When the pattern sees \r\n will it match one time or twice?
And what is the rules face to this situation?

Thanks

Answer 1

It will match \r\n once because Perl uses a regex-directed engine which evaluates alternations eagerly. See here.

You can easily find out whether the regex flavor you intend to use has a text-directed or regex-directed engine. If backreferences and/or lazy quantifiers are available, you can be certain the engine is regex-directed. You can do the test by applying the regex regex|regex not to the string regex not. If the resulting match is only regex, the engine is regex-directed. If the result is regex not, then it is text-directed. The reason behind this is that the regex-directed engine is “eager”.