regex - How to match a new line character in Python raw string

Question

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' ' 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 1

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

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

Notice that re translates the (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
dog',s).group(0)
'cat
dog'
>>> re.match(r'^cat$
dog',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$
dog',s,re.M).group(0) #matches.
'cat
dog'