How to remove string except followed by certain characters with regular expression?

Question

I want to remove ab from a string if ab is not followed by x or y.

For example, if the string is 123ab456, the result should be 123456.

If the string is 123abx456, the result should be123abx456.

How could I use regex to do so?

Answer 1

Here's a way using re.sub with a negative lookahead:

re.sub(r'ab(?![xy])', '', s)

---

s = '123ab456'
re.sub(r'ab(?![xy])', '', s)
# '123456'

s = '123abx456'
re.sub(r'ab(?![xy])', '', s)
# '123abx456'

---

Details

• ab(?![xy])
  • ab matches the characters ab literally (case sensitive)
  • Negative Lookahead (?![xy])
    • Match a single character present in the list [xy]
    • xy matches a single character in the list xy (case sensitive)