I think the problem you’re seeing is probably caused by failure to match \b. From “Assertions” in perlre#Regular-Expressions:

A word boundary (\b) is a spot between two characters that has a \w on one side of it and a \W on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a \W.

So, if there is there is no underscore character, the expression (?:\b|_) attempts to match \b, but this fails if $parse1 begins with a forward slash since that character is not a “word” character — unless the preceding character happens to be a “word” character, which is unlikely in this scenario.

Hope that helps,