Python 正则表达式前瞻断言

Question

I want to get word1 and word2 from text string like text = "otherstuff word1 + word2" .

I managed to get one word using this lookbehind regular expression:

r1 = re.search(r'(?<=\+\s)\w+', text) # Get word2

But whenever I try to do the same with the lookahead, regex it doesn't return anything

r2 = re.search(r'(?=(\s\+))\w+', text) # Get None

What did I do wrong?

Answer 1

Could you try this? Just swapped position of your \w+ and (?=(\s\+)) .

r2 = re.search(r'\w+(?=(\s\+))', text)

Answer 2

What did I do wrong?

This pattern (?=(\s\+))\w+ asserts what is directly to the right from the current position should be a whitespace char \s but then you start the match with a word char \w which will not match.

You could get both words by using a single pattern with an alternation | and switch the order of the lookaround.

(?<=\+\s)\w+|\w+(?=\s\+)

Regex demo

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To get both words, you could also use 2 capturing groups instead of lookarounds:

(\w+)\s\+\s(\w+)

Regex demo

import re

pattern = r"(\w+)\s\+\s(\w+)"
s = "otherstuff word1 + word2."
print(re.findall(pattern, s))

Output

[('word1', 'word2')]