Regex | Positive Lookbehind

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Example

You have a test String S.
Write a regex which can match all the occurences of digit which are immediately preceded by odd digit.
Input: 123Go!
Output: Number of matches : 1

Regex_Pattern = r"(?<=[1,3,5,7,9])\d"

import re

Test_String = input()

match = re.findall(Regex_Pattern, Test_String)

print("Number of matches :", len(match))

Theory

(?<=regex_2)regex_1
The positive lookbehind (?<=) asserts regex_1 to be immediately preceded by regex_2. Lookbehind is excluded from the match (do not consume matches of regex_2), but only assert whether a match is possible or not.


Source: https://yellorn.com/programming/regex-positive-lookbehind

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