Python Regular Expression Problem with Numbers

Question

I need to find the space after 3 or 4 digits in a bunch of filenames and replace the space with an underscore. But I can't seem to even find 4 digits together.

s = "the blue dog and blue cat wore blue hats"
p = re.compile(r'blue (?P<animal>dog|cat)')
print(p.sub(r'gray \g<animal>',s))

#Gives basically what I want.
the gray dog and gray cat wore blue hats


s = "7053 MyFile.pptx"
p = re.compile('[0-9][0-9][0-9][0-9](?P<dig> )')
print(p.sub('_\g<dig>', s))

#Takes out the numbers, which I need to keep
_ MyFile.pptx

Everything I seem to do has the expression taking out the digits, which I need to keep.

In the end, I want

7035 MyFile.pptx

to be

7035_MyFile.pptx

Why overcomplicate a simple task by using regex? You can just split on the whitespace and join the resulting list on "_". — Fynn Becker
– Fynn Becker, Commented Jan 14, 2019 at 21:28
There are other file names with white space in it. 1234 Some Other File.pptx Plus I need to get better with my regex :) — James Holland
– James Holland, Commented Jan 14, 2019 at 21:36
That's still no issue as long as the filenames start with the digits. But sure you can a use regex for this task; I'm just recommending not to if this goes beyond personal projects/exploration. — Fynn Becker
– Fynn Becker, Commented Jan 14, 2019 at 21:38

Ondrej K. · Accepted Answer · 2019-01-14 21:42:35Z

I you want to replace 3 or 4 digits followed by a white space with the same digit followed by an underscore, the correct regex syntax/substitution would be:

re.sub(r"([0-9]{3,4})\s", r"\1_", s)

You might have misread how the groups/back references work. What is supposed to be in the group, needs to be inside the parenthesis. If you wanted to use a named group (which is a bit unnecessary):

re.sub(r"(?P<dig>[0-9]{3,4})\s", r"\g<dig>_", s)

Or with a pre-compiled regex akin to your example:

s = "7053 MyFile.pptx"
p = re.compile(r"(?P<dig>[0-9]{3,4})\s")
print(p.sub('\g<dig>_', s))

{3,4} following [0-9] means three or four matches. \s stands for a white space (not just a space).

Actually just looking for 3 digits as written would also match 4 digits, because we do not limit what happens before the matching pattern. Depending on what you are looking for, you may want to limit the matches by prepending the pattern with ^ (beginning of line) or \b empty character at word edge...

Collectives™ on Stack Overflow

Python Regular Expression Problem with Numbers

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related