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

Question

Try this code.

test = ' az z bz z z stuff z  z '
re.sub(r'(W)(z)(W)', r'1_23', 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?

Answer 1

If your goal is to make sure you only match z when it's a standalone word, use to match word boundaries without actually consuming the whitespace:

>>> re.sub(r'(z)', r'_1', test)
' az _z bz _z _z stuff _z  _z '