Regular Expression to find a string included between two characters

 Base string: This is a test string [more or less]

need to get only more or less (without the brackets). 

Easy done:

(?<=\[)(.*?)(?=\])

Technically that's using lookaheads and lookbehinds. See Lookahead and Lookbehind Zero-Width Assertions. The pattern consists of:

• is preceded by a [ that is not captured (lookbehind);
• a non-greedy captured group. It's non-greedy to stop at the first ]; and
• is followed by a ] that is not captured (lookahead).

Alternatively you can just capture what's between the square brackets:

\[(.*?)\]

and return the first captured group instead of the entire match.

Source: https://stackoverflow.com/questions/1454913/regular-expression-to-find-a-string-included-between-two-characters-while-exclud