Question

I’m trying to use this regular expression, but I’m having trouble testing it, because for whatever reason it is crashing the only two regex testers I use.

^([a-zA-Z-]+/?)+$

If I’m not mistaken, that should match any URI that follows this format:

some-uri-string/some-uri-string/some-uri-string

However, I don’t want it to match:

some-uri-string/some-uri-string//some-uri-string

Notice the double slash. Essentially, it is category/sub-category/sub-sub-category/page

With any number of sub-categories being allowed.

Answer 1

The problem is that, for a non-matching string, your pattern is a pathological case for a backtracking regular expression engine (which is true of those in most scripting languages, like Perl, Python, and anything based on PCRE).

More information:

• http://swtch.com/~rsc/regexp/regexp1.html
• http://www.regular-expressions.info/catastrophic.html

You could rewrite it as:

^([a-zA-Z-]+/)*[a-zA-Z-]+$ 

Or your RE implementation may have other facilities available to avoid that scenario.