[theamk]

A lot of time I want to check for valid identifier:

    if not re.match('^[a-z0-9_]+$', user):
        raise SomeException("invalid username")

as written, the code above is incorrect - it will happily accept "john\n", which can cause all sort of havoc down the line

[extraduder_ire]

Shouldn't you use the match returned from the string? Or use .fullmatch() (added 3.4) to match the whole string.

[theamk]

In general no, you should not use match from the string. If you are getting input from user, you want a more complex processing (like stripping all whitespace), and if you are getting input from API calls, you want to either use specified name as-is, or fail.

Yes, fullmatch() will help, and so will \Z. It's just that it is so easy to forget...