How can I modify my regex to avoid replacing 'ABC' when followed by '.png' or ' thumb.png'?

This will replace 'ABC' with 'bueno' only if it is not followed by '.png' or ' thumb.png'.

PYTHONCopied
import re

texto = "ABC ABC. ABC.png ABC thumb.png"

regex = r"\bABC\b(?!\.png|\s?thumb\.png)"

novo = re.sub(regex, "bueno", texto)

print(novo)

RESULT:

bueno bueno. ABC.png ABC thumb.png

NOTE: In the regex, added \b at the end of ABC => \bABC\b, so that it will not match if there is another word character after the letter C. For example, it will not match 'ABCd', or *'zABCd'.

You can make thumb optional inside the lookahead:

REGEXCopied
\bABC(?!(?: thumb)?\.png)

The regex matches:

• \bABC Match ABC preceded by a word boundary to prevent a partial word match
• (?! Negative lookahead, assert that what is directly to the right is not
  • (?: thumb)? Optionally match thumb
  • \.png Match .png
• ) Close the lookahead

See a regex 101 demo

Note that if .png should also not have a partial word match at the end, you can also a word boundary like .png\b

The OR (|) needs to be inside the negative lookahead:

REGEXCopied
ABC(?!.png| thumb.png)

Let's take the substring ABC.png as an example of what was happening with your original pattern. The regex engine would first try to match this using ABC(?!.png), which would fail because of the negative lookahead. Then, because of the OR, it would jump to the next case, ABC(?! thumb.png). This would match ABC.png, which means the string would get replaced.

Starting with your original pattern:

REGEXCopied
ABC(?!\.png)|ABC(?! thumb\.png)
(Note: Dot is a regex metacharacter and should be escaped with backslash)

This will match ABC which is not followed by .png or ABC not followed by thumb.png. Every possible occurrence of ABC will match this pattern. Therefore, all occurrences of ABC will be match, because every extension will match at least one of the two conditions.

We can write the following correction:

REGEXCopied
\bABC(?!\.png| thumb\.png)

This pattern says to match:

• \b word boundary
• ABC match ABC
• (?!\.png| thumb\.png) neither .png or thumb.png follows

The negative lookahead used here basically has AND flavored logic, and will exclude both following extensions.