Beginning (python) regex question

Question

I'm having trouble figuring out why the following regex doesn't seem to work.

I know that I can form other regular expressions to make this work, but I thought this one should work.

re.search ("(\d*)", "prefix 1234 suffix").groups()
('',)

Interestingly, findall seems to work:

re.findall("(\d*)", "prefix 1234 suffix")
['', '', '', '', '', '', '', '1234', '', '', '', '', '', '', '', '']

I understand why that works, but I'm still confused as to why search doesn't work? My understanding is that match should force it to match the whole string, but search should find the digits anywhere within the string

hobbs · Accepted Answer · 2011-01-24 14:44:33Z

6

Because .search runs the search once, and matches in the first place it can. Since \d* can match no characters at all, the first place it can match is at the beginning of the string, capturing no characters -- so the first capture group is ''. It's doing exactly what you asked it to.

If you had made the regex (\d+) instead, which has to match at least one digit, then the first place it could match would be at the 1 and it would capture 1234.

answered Jan 24, 2011 at 14:44

hobbs

245k20 gold badges225 silver badges304 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

detunized · Accepted Answer · 2011-01-24 14:46:16Z

3

Use \d+, not \d*. \d* means zero or more, and that zero is what you get at the offset 0 in the string.

answered Jan 24, 2011 at 14:46

detunized

15.3k3 gold badges50 silver badges66 bronze badges

Comments

SilentGhost · Accepted Answer · 2011-01-24 14:50:23Z

3

It works. Return values of your first example corresponds to the first element of return value of findall. Just use: r'(\d+)' as your regex.

search does find digits anywhere within the string, it just your regex that tells to find digits zero or more times. So it finds, zero digits at every character border.

edited Jan 24, 2011 at 14:50

answered Jan 24, 2011 at 14:43

SilentGhost

322k67 gold badges312 silver badges294 bronze badges

Comments

Andrew Hare · Accepted Answer · 2011-01-24 14:43:04Z

1

Try this:

re.findall("(\d+)", "prefix 1234 suffix")

By changing the * to a + you are indicating that the pattern \d must match 1 or more times. The * you were using at first was matching zero or more times which was matching each and every character in the string.

answered Jan 24, 2011 at 14:43

Andrew Hare

353k75 gold badges649 silver badges642 bronze badges

1 Comment

Hortitude Over a year ago

I added a little to my question to clarify my confusion. I understand that findall would work -- but was wondering why search doesn't work.

Collectives™ on Stack Overflow

Beginning (python) regex question

4 Answers 4

Comments

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest