I'm trying to write a regex to match part of a string that comes before '/' but also ignores any leading or trailing white space within the match.

So far I've got ^[^\/]* which matches everything before the '/' but I can't figure out how to ignore the white space.

      123 / some text 123

should yield

123

and

     a test / some text 123

should yield

a test

That's a little bit tricky. You first start matching from a non-whitespace character then continue matching slowly but surely up to the position that is immediately followed by an optional number of spaces and a slash mark:

\S.*?(?= *\/)

See live demo here

If slash mark could be the first non-whitespace character in input string then replace \S with [^\s\/]:

[^\s\/].*?(?= *\/)