Trouble with Regex backreference in Python

Question

I'm playing around with group backreferences in Python's Regex to try to understand them and I'm not having much luck.

import re

leftQuotes = re.compile("((\"|\“)([\w|\d]))")
rightQuotes = re.compile("(([\w|\d])(\"|\”))")

s = "This is “problematic”"

s = re.sub(leftQuotes, r'‘\3', s)
s = re.sub(rightQuotes, r'’\3', s)

print(s)

Output:

This is ‘problemati’”

In the first re.sub(), I managed to successfully replace the left double quotation mark with a single left quotation mark while keeping the matching character (in this case, a "p"). But the right side doesn't behave in the same way, regardless of the group backreference (1, 2, 3).

Results of backreferences:

\1: ‘problemati’c” 
\2: ‘problemati’c 
\3: ‘problemati’”

You overcaptured it. Use s = re.sub(rightQuotes, r'\2’', s), or better just remove unnecessary groups to only keep one that you need to keep, then just use Group 1 backreference. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 29, 2017 at 19:59
@WiktorStribiżew As I'd said originally in the post, all backreferences resulted in undesirable output. \1 gets This is ‘problemati’c” — David Metcalfe
– David Metcalfe, Commented Sep 29, 2017 at 20:03

akuiper · Accepted Answer · 2017-09-29 20:19:22Z

2

To fix your code, replace the second sub with:

s = re.sub(rightQuotes, r'\2’', s)

should work, since the word character in the second pattern comes as the second capture group and it should come before the single quote replacement as well.

Besides, you don't really need capture groups here, use look around would be cleaner, (though not critical quoting the string with single quote can save you some typing as @CasimiretHippolyte's comment):

import re

leftQuotes = re.compile('(?:"|“)(?=\w)')
rightQuotes = re.compile('(?<=\w)(?:"|”)')

s = "This is “problematic”"

s = re.sub(leftQuotes, r'‘', s)
s = re.sub(rightQuotes, r'’', s)

s
# 'This is ‘problematic’'

Also since \w includes \d, [\w|\d] can be replaced by \w.

edited Sep 29, 2017 at 20:19

answered Sep 29, 2017 at 20:07

akuiper

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3 Comments

Casimir et Hippolyte Over a year ago

Instead of a lookahead or a lookbehind, it's more simple to use a word boundary.

akuiper Over a year ago

@CasimiretHippolyte Good call.

Casimir et Hippolyte Over a year ago

I meant something like : (?:"|“)\b and \b(?:"|“)

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