CODEWITHSUNDEEP
pythonregexstringmatching
npross
npross

Reputation: 1888

Python string matching, error: nothing to repeat at position 0

I have a column called `cc_flags' in a Python astropy table. It looks like::

cc_flags

0000
ddOO
0000
hHOO
0000
DD00
hHOO
hHPO
P000
00h0
...
0000

I'd like to pull out every row with the last character of the four if it is a '0'., i.e a '***0' or a '???0' matcher.

I'm getting an 

error: nothing to repeat at position 0

with 

import re
regex = r"???0"
re.findall(regex,data['cc_flags'])

What am I doing wrong?!

Upvotes: 0

Views: 2725

Answers (4)

salomonvh
salomonvh

Reputation: 1809

The issue is with regex not escaping the special characters in your search term. To get this to work, simply change your statement to the following:

import re
regex = re.escape("???0")
re.findall(regex,data['cc_flags'])

data['cc_flags'] should be a string, not a list.

Upvotes: 0

pault
pault

Reputation: 43534

As others have said, you need to use . instead of ?. Another way is to use {} to specify how many matches.

For example, try this:

import re
regex = r".{3}0"
re.findall(regex,data['cc_flags'])
  • . means match any character (except for line terminators)
  • {3} means match exactly 3 times

Upvotes: 1

Scott Mermelstein
Scott Mermelstein

Reputation: 15397

If you're not limited to regex or regex like, this is simple:

has_zeros = [line for line in data['cc_flags'] if line[-1] == '0']

This list comprehension will extract out each element from data['cc_flags'] into a variable named line, and return a resulting list that is only comprised of the last character of line being a 0.

(Obviously, if you need your if statement to be more specific, you can add whatever else you need into the filter condition.)

Upvotes: 2

Simon Brahan
Simon Brahan

Reputation: 2086

? does not mean "any character" in regex. Swap your ?s for .s

Upvotes: 3

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