[python]: problem about python string literals

Question

code goes below:

line = r'abc\def\n'
rline = re.sub('\\\\', '+', line) # then rline should be r'abc+def+n'

Apparently, I just want to replace the backslashes in line with '+'. What I thought was that a backslash in line can be expressed as '\', then why should I use '\\' to get the re.sub work right.

I'm confused.

@nagisa -- ya, that's a way circumventing the problem, but I want to know why re.sub doesn't work that way. — Alcott
– Alcott, Commented Aug 16, 2011 at 12:53

unutbu · Accepted Answer · 2011-08-16 11:25:23Z

7

It's a good habit to always use raw strings when dealing with regex patterns:

In [45]: re.sub(r'\\', r'+', line)
Out[45]: 'abc+def+n'

To answer your question though, Python interprets '\\\\' as two backslash characters:

In [44]: list('\\\\')
Out[44]: ['\\', '\\']

And the rules of regex interpret two backslash characters as one literal backslash.

answered Aug 16, 2011 at 11:25

unutbu

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UncleZeiv · Accepted Answer · 2011-08-16 11:42:59Z

4

Because there are two levels of backslashing:

re.sub uses \ as an escape
Python uses \ as an escape (unless you do r'...')

So \\\\ (python) -> \\ (re.sub) -> \

EDIT

And the SO level of backslashing! (it got me!)

edited Aug 16, 2011 at 11:42

UncleZeiv

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answered Aug 16, 2011 at 11:26

Owen

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6 Comments

agf Over a year ago

+1 for cleverness -1 for not using backticks to format code properly. :P.

UncleZeiv Over a year ago

formatted with backticks, doesn't require additional backslashing ;)

agf Over a year ago

I didn't edit it since he made a point of getting it "right" without.

Alcott Over a year ago

@Owen -- It seems like your answer pretty much convince me. Hah.

Alcott Over a year ago

and one more question, what do you mean by SO level ?

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millimoose · Accepted Answer · 2011-08-16 11:37:38Z

2

If you want to search for a literal pattern, not an actual regular expression, you should use both raw strings and re.escape() to avoid doubling backslashes or any other manual escaping completely.

So, your example would become:

line = r'abc\def\n'
backslash = re.escape(r'\')
rline = re.sub(backslash, '+', line)

answered Aug 16, 2011 at 11:37

millimoose

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1 Comment

millimoose Over a year ago

Hmm, apparently the SO syntax highlighter doesn't like raw strings.

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