Question

I’m looking for data that looks like abc/def. There may be whitespace around the text anywhere, so all of the following would be valid: abc__/_def or __abc/__def__. (Using underscores to visualize spaces.)

I came up with this regular expression:

(?<=\s*)abc\s*\/\s*def(?=\s*|^)

This works to find matches. I only recently picked up look-ahead and tried this expression to exclude spaces around / from the match (so abc__/_def would produce a match abc/def):

(?<=\s*)abc(?=\s*)\/(?=\s*)def(?=\s*|^)

This expression doesn’t work – I obviously misunderstand something about look-aheads. Can someone explain the difference between the two expressions? (Is it even possible what I’m trying to do? After reading the Regex documentation I thought it was but maybe I’m wrong.)

Answer 1

The match returned by any .NET regex is a contiguous substring of the original string. This means that you cannot get rid of the space around the “/” character. You can get rid of the outer spaces though.

A good approach would be to match abc and def using named groups and extract that information using

Match m = ...;
var part1 = m.Groups["part1"].Value;
var part2 = m.Groups["part2"].Value;

Try this: ^\s*(?<part1>\w+)\s*/\s*(?<part2>\w+)\s*$

Oftentimes the regex is more natural with named groups and without lookaround.