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'\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?

1
27
2/4/2013 3:24:01 PM

Accepted Answer

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'
27
2/4/2013 3:39:33 PM

The simplest answer is to simply not use a raw string. You can escape backslashes by using \\.

If you have huge numbers of backslashes in some segments, then you could concatenate raw strings and normal strings as needed:

r"some string \ with \ backslashes" "\n"

(Python automatically concatenates string literals with only whitespace between them.)

Remember if you are working with paths on Windows, the easiest option is to just use forward slashes - it will still work fine.


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