They say that if you have a problem and want to use RegEx to solve it, then you have two problems. So true! 🙂

My specific problem was that I wanted to search for a string within a substring after a delimiter sign, more precisely, in the last segment of a path. Here is an example:

/some/test_path/to/search/with_a_Test_file.txt

The RegEx, searching without case sensitivity for “test” should return a match only for the portion of the string after the last “/”.
All suggestions, which I could find on StackOverflow, concerned with matching the entire file name and not a portion of it, so I had to learn some advanced RegEx. Fast.

The answer was something, called “lookahead”, which is well explained at Regular-Expressions.info site.

The resulting RegExt string looks like some serious swearing in a cartoon bubble… 🙂 Here is the code, which is accepted by PHP’s preg_match() function:

/(?=[^\/]+$)test/i

According to my (rather limited) understanding of RegEx, the first portion in the parenthesis, after the “?=”is the lookahead, which matched the entire file name after the last “/”, then comes the search substring, “test”, which operates on that result and, finally, “/i” is the switch, instructing a case-insensitive match.