python regex can not match if newline exists in string?

Question

I am using regex to find time in a string:

re.match('.*(\d{2}-\d{2} \d{2}:\d{2})', "09-22 13:27")

anyway, this is ok, but if \n exists, this return None:

re.match('.*(\d{2}-\d{2} \d{2}:\d{2})', "\n09-22 13:27")

so why .* can not match \n? and how to deal with this?

See docs.python.org/2/library/re.html#re.DOTALL "why .* can not match \n" - because it's not supposed to. — jonrsharpe
– jonrsharpe, Commented Sep 29, 2015 at 11:29
Anyway, . matches any symbol but a newline. Use re.S flag to match a newline with . — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 29, 2015 at 11:30

Wiktor Stribiżew · Accepted Answer · 2015-09-29 11:32:57Z

4

why .* can not match \n? and how to deal with this?

re documentation says:

re.DOTALL
Make the . special character match any character at all, including a newline; without this flag, . will match anything except a newline.

You should note that . matches any symbol but a newline. Use re.S (re.DOTALL) flag to match a newline with .:

import re
obj = re.match('.*(\d{2}-\d{2} \d{2}:\d{2})', "\n09-22 13:27", re.S)
if obj:
    print(obj.group(1))

See IDEONE demo

answered Sep 29, 2015 at 11:32

Wiktor Stribiżew

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3 Comments

juan Isaza Over a year ago

in case I'am using re.sub, I tried adding the flag but didn't work. Ended up with a clumsy solution re.finall + replace. Is there a better way?

Wiktor Stribiżew Over a year ago

@juanIsaza Do you mean you are using something like re.sub('(.*)\d{2}-\d{2} \d{2}:\d{2}', r"\g<1>09-22 13:27", flags=re.S)?

juan Isaza Over a year ago

sorry, was missing the flags argument.

dhh · Accepted Answer · 2015-09-29 12:47:16Z

1

You can exclude newline by using (?

st =  "\n\n09-22 13:27\n"
import re
mo = re.findall(r'(?<!\S)\d{2}-\d{2}\D+\d{2}:\d{2}(?!\S)',st)
print(mo)

Results in:

['09-22 13:27']

edited Sep 29, 2015 at 12:47

dhh

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answered Sep 29, 2015 at 11:49

LetzerWille

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Comments

vks · Accepted Answer · 2015-09-29 11:34:22Z

0

print re.match('.*(\d{2}-\d{2} \d{2}:\d{2})', "\n09-22 13:27".strip())

                                                              ^^^^^^^^

You can simply strip the newline.

answered Sep 29, 2015 at 11:34

vks

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2 Comments

Ray Over a year ago

What if the string necessarily contains new lines? For example, if it's read from a text file and for some reason you don't want to modify it at all when matching? What if your regex pattern depends on new line characters?

vks Over a year ago

by default . matches any character (except for line terminators)

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