CODEWITHSUNDEEP
pythonregex
goh
goh

Reputation: 29561

regex error - nothing to repeat

I get an error message when I use this expression:

re.sub(r"([^\s\w])(\s*\1)+","\\1","...")

I checked the regex at RegExr and it returns . as expected. But when I try it in Python I get this error message:

raise error, v # invalid expression
sre_constants.error: nothing to repeat

Can someone please explain?

Upvotes: 98

Views: 221698

Answers (6)

robertspierre
robertspierre

Reputation: 4431

I had this problem when using the regex \b?. Using \s? fixed the issue (although it's not the same thing)

Upvotes: 0

Ayoub Arroub
Ayoub Arroub

Reputation: 495

regular expression normally uses * and + in theory of language. I encounter the same bug while executing the line code 

re.split("*",text)

to solve it, it needs to include \ before * and +

re.split("\*",text)

Upvotes: 14

nealmcb
nealmcb

Reputation: 13481

Beyond the bug that was discovered and fixed, I'll just note that the error message sre_constants.error: nothing to repeat is a bit confusing. I was trying to use r'?.*' as a pattern, and thought it was complaining for some strange reason about the *, but the problem is actually that ? is a way of saying "repeat zero or one times". So I needed to say r'\?.*'to match a literal ?

Upvotes: 10

Ando Jurai
Ando Jurai

Reputation: 1049

It's not only a Python bug with * actually, it can also happen when you pass a string as a part of your regular expression to be compiled, like ;

import re
input_line = "string from any input source"
processed_line= "text to be edited with {}".format(input_line)
target = "text to be searched"
re.search(processed_line, target)

this will cause an error if processed line contained some "(+)" for example, like you can find in chemical formulae, or such chains of characters. the solution is to escape but when you do it on the fly, it can happen that you fail to do it properly...

Upvotes: 13

Franklyn
Franklyn

Reputation: 251

That is a Python bug between "*" and special characters.

Instead of

re.compile(r"\w*")

Try:

re.compile(r"[a-zA-Z0-9]*")

It works, however does not make the same regular expression.

This bug seems to have been fixed between 2.7.5 and 2.7.6.

Upvotes: 25

mb14
mb14

Reputation: 22616

It seems to be a python bug (that works perfectly in vim). The source of the problem is the (\s*...)+ bit. Basically , you can't do (\s*)+ which make sense , because you are trying to repeat something which can be null.

>>> re.compile(r"(\s*)+")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/re.py", line 180, in compile
    return _compile(pattern, flags)
  File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/re.py", line 233, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

However (\s*\1) should not be null, but we know it only because we know what's in \1. Apparently python doesn't ... that's weird.

Upvotes: 59

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