Question

I am very weak at regular expressions, now I’m debugging some code, the code is searching strings with an expression like:

r"coding[:=]\s*([-\w.]+)"

What kind of string does it search for?

To me, it seems to match something like:

coding= xxxxx

but I don’t know the exact meaning of the mystery character. Can anyone explain in a bit more detail?

Answer 1

Let’s break this down:

• coding: literal text match, only the word “coding” will do
• [:=]: character group, either a colon “:” or an equals sign “=” matches
• \s*: 0 or more whitespace characters; spaces and tabs, but could match newlines too if so configured.
• (..): a matching group, the contents will be available as a match group for further processing.
• [-\w.]+: one or more characters in the group, matching a dash “-“, a dot “.” or any word character; \w is a character class usually matching the letters ‘a’ through ‘z’ (upper and lowercase), numbers ‘0’ up to ‘9’ and the underscore “_”.

If you switch on unicode support (on by default in Python 3) the \w class captures a lot more though. Any character classified as an alphanumeric in the Unicode database would match.

Examples of matches:

• “coding=foobar320_42spam_eggs”, group would be foobar320_42spam_eggs
• “coding: something-or-other”, group would be something-or-other
• “coding:       whatever.42”, group would be whatever.42