为什么 \w+ 匹配尾随换行符？

Question

I am curious why the following would output that there was a match:

import re

foo = 'test\n'
match = re.search('^\w+$', foo)

if match == None:
  print "It did not match"
else:
  print "Match!"

The newline is before the end of the string, yes? Why is this matching?

Answer 1

From Python's re documentation.

'$'
Matches the end of the string or just before the newline at the end of the string, and in MULTILINE mode also matches before a newline. foo matches both 'foo' and 'foobar', while the regular expression foo$ matches only 'foo'. More interestingly, searching for foo.$ in 'foo1\nfoo2\n' matches 'foo2' normally, but 'foo1' in MULTILINE mode; searching for a single $ in 'foo\n' will find two (empty) matches: one just before the newline, and one at the end of the string.

Answer 2

^ and $ mean "start of line" and "end of line", not "start of string" and "end of string". Use \A for "start of string" and \Z for "end of string".