Python regex - why does end of string ($ and \Z) not work with group expressions?

Question

In Python 2.6. it seems that markers of the end of string $ and \Z are not compatible with group expressions. Fo example

import re
re.findall("\w+[\s$]", "green pears")

returns

['green ']

(so $ effectively does not work). And using

re.findall("\w+[\s\Z]", "green pears")

results in an error:

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/re.pyc in findall(pattern, string, flags)
    175 
    176     Empty matches are included in the result."""
--> 177     return _compile(pattern, flags).findall(string)
    178 
    179 if sys.hexversion >= 0x02020000:

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/re.pyc in _compile(*key)
    243         p = sre_compile.compile(pattern, flags)
    244     except error, v:
--> 245         raise error, v # invalid expression
    246     if len(_cache) >= _MAXCACHE:
    247         _cache.clear()

error: internal: unsupported set operator

Why does it work that way and how to go around?

Answer 1

A [..] expression is a character group, meaning it'll match any one character contained therein. You are thus matching a literal $ character. A character group always applies to one input character, and thus can never contain an anchor.

If you wanted to match either a whitespace character or the end of the string, use a non-capturing group instead, combined with the | or selector:

r"\w+(?:\s|$)"

Alternatively, look at the \b word boundary anchor. It'll match anywhere a \w group start or ends (so it anchors to points in the text where a \w character is preceded or followed by a \W character, or is at the start or end of the string).