如果模式后面跟着另一个模式，正则表达式应该会失败

Question

I need to detect @username mentions within a message, but NOT if it is in the form of @username[user_id] . I have a regex that can match the @username part, but am struggling to negate the match if it is followed by \[\d\] .

import re

username_regex = re.compile(r'@([\w.@-]+[\w])')

usernames = username_regex.findall("Hello @kevin") # correctly finds kevin
usernames = username_regex.findall("Hello @kevin.") # correctly finds kevin
usernames = username_regex.findall("Hello @kevin[1].") # shouldn't find kevin but does

The regex allows for usernames that contain @ , . and - , but need to end with a \w character ( [a-zA-Z0-9_] ). How can I extend the regex so that it fails if the username is followed by the userid in the [1] form?

I tried @([\w.@-]+[\w])(?!\[\d+\]) but then it matches kevi

I'm using Python 3.10.

Answer 1

You can "emulate" possessive matching with

@(?=([\w.@-]*\w))\1(?!\[\d+\])

See the regex demo .

Details :

• @ - a @ char
• (?=([\w.@-]*\w)) - a positive lookahead that matches and captures into Group 1 zero or more word, . , @ and - chars, as many as possible, and then a word char immediately to the right of the current position (the text is not consumed, the regex engine index stays at the same location)
• \1 - the text matched and captured in Group 1 (this consumes the text captured with the lookahead pattern, mind that backreferences are atomic by nature)
• (?!\[\d+\]) - a negative lookahead that fails the match if there is [ + one or more digits + ] immediately to the right of the current location.