How to match a new line character in Python raw string

python regex rawstring
wei picture wei · Feb 4, 2013 · Viewed 109.3k times · Source

I got a little confused about Python raw string. I know that if we use raw string, then it will treat '\' as a normal backslash (ex. r'\n' would be \ and n). However, I was wondering what if I want to match a new line character in raw string. I tried r'\\n', but it didn't work.

Anybody has some good idea about this?

Answer

mgilson picture mgilson · Feb 4, 2013

In a regular expression, you need to specify that you're in multiline mode:

>>> import re
>>> s = """cat
... dog"""
>>> 
>>> re.match(r'cat\ndog',s,re.M)
<_sre.SRE_Match object at 0xcb7c8>

Notice that re translates the \n (raw string) into newline. As you indicated in your comments, you don't actually need re.M for it to match, but it does help with matching $ and ^ more intuitively:

>> re.match(r'^cat\ndog',s).group(0)
'cat\ndog'
>>> re.match(r'^cat$\ndog',s).group(0)  #doesn't match
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'
>>> re.match(r'^cat$\ndog',s,re.M).group(0) #matches.
'cat\ndog'

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