Why does re.sub in Python not work correctly on this test case?

python, regex

Solution

The reason why it does that is that you get an overlapping match; you need to not match the extra character - there are two ways you can do this; one is using `\b`, the word boundary, as suggested by others, the other is using a lookbehind assertion and a lookahead assertion. (If reasonable, as it should probably be, use `\b` instead of this solution. This is mainly here for educational purposes.)

>>> re.sub(r'(?<!\w)(z)(?!\w)', r'_\1', test)
' az _z bz _z _z stuff _z  _z '

`(?<!\w)` makes sure there wasn't `\w` before.

`(?!\w)` makes sure there isn't `\w` after.

The special `(?...)` syntax means they aren't groups, so the `(z)` is `\1`.

As for a graphical explanation of why it fails:

The regex is going through the string doing replacement; it's at these three characters:

' az _z bz z z stuff z  z '
          ^^^

It does that replacement. The final character has been acted upon, so its next step is approximately this:

' az _z bz _z z stuff z  z '
              ^^^ <- It starts matching here.
             ^ <- Not this character, it's been consumed by the last match

Problem

Try this code. ``` test = ' az z bz z z stuff z z ' re.sub(r'(\W)(z)(\W)', r'\1_\2\3', test) ``` This should replace all stand-alone z's with _z However, the result is: ' az _z bz _z z stuff _z _z ' You see there's a z there that is missing. I theorize that it's because the grouping can't grab the space between the z's to match two z's at once (one for trailing whitespace, one for leading whitespace). Is there a way to fix this?

Original source