'\\' in regex affects the following parenthesis?

[John Machin]

On Apr 22, 9:56 am, vox... at gmail.com wrote:
> Could someone tell me why:
>
> >>> import re
> >>> p = re.compile('\\.*\\(.*)')

Short answer: *ALWAYS* use raw strings for regexes in Python source
files.

Long answer:

'\\.*\\(.*)' is equivalent to
r'\.*\(.*)'

So what re.compile is seeing is:

\. -- a literal dot or period or full stop (not a metacharacter)
* -- meaning 0 or more occurrences of the dot
\( -- a literal left parenthesis
. -- dot metacharacter meaning any character bar a newline
* -- meaning 0 or more occurences of almost anything
) -- a right parenthesis grouping metacharacter; a bit lonely hence
the exception.

What you probably want is:

\\ -- literal backslash
.* -- any stuff
\\ -- literal backslash
(.*) -- grouped (any stuff)


>
> Fails with message:
>
> Traceback (most recent call last):
>   File "<pyshell#12>", line 1, in <module>
>     re.compile('\\dir\\(file)')
>   File "C:\Python25\lib\re.py", line 180, in compile
>     return _compile(pattern, flags)
>   File "C:\Python25\lib\re.py", line 233, in _compile
>     raise error, v # invalid expression
> error: unbalanced parenthesis
>
> I thought '\\' should just be interpreted as a single '\' and not
> affect anything afterwards...

The second and third paragraphs of the re docs (http://docs.python.org/
lib/module-re.html) cover this:
"""
Regular expressions use the backslash character ("\") to indicate
special forms or to allow special characters to be used without
invoking their special meaning. This collides with Python's usage of
the same character for the same purpose in string literals; for
example, to match a literal backslash, one might have to write '\\\\'
as the pattern string, because the regular expression must be "\\",
and each backslash must be expressed as "\\" inside a regular Python
string literal.

The solution is to use Python's raw string notation for regular
expression patterns; backslashes are not handled in any special way in
a string literal prefixed with "r". So r"\n" is a two-character string
containing "\" and "n", while "\n" is a one-character string
containing a newline. Usually patterns will be expressed in Python
code using this raw string notation.
"""

Recommended reading: http://www.amk.ca/python/howto/regex/regex.html#SECTION000420000000000000000

>
> The script 'redemo.py' shipped with Python by default is just fine
> about this regex however.

That's because you are typing the regex into a Tkinter app. Likewise
if you were reading the regex from (say) a config file or were typing
it to a raw_input call. The common factor is that you are not passing
it through an extra level of backslash processing.

HTH,
John