Python Regular Expression: why does this not work?

问题

This does not give me an error nor an answer.

re.sub('\\.(\\W|\\.)*[o0](\\W|[o0])*', '*', '..........................................')

Why does it behave like so? Also, if I reduce the amount of 'periods', then it works.

Thank you.

回答1:

You've got catastrophic backtracking.

回答2:

You have no o or 0 in your input string, yet your regular expression requires at least one of those characters to be there ([o0]).

>>> re.compile('\\.(\\W|\\.)*[o0](\\W|[o0])*', re.DEBUG)
literal 46
max_repeat 0 65535
  subpattern 1
    branch
      in
        category category_not_word
    or
      literal 46
in
  literal 111
  literal 48
max_repeat 0 65535
  subpattern 2
    branch
      in
        category category_not_word
    or
      in
        literal 111
        literal 48

Update: Your regular expression is suffering from catastrophic backtracking; avoid the nested character-class-or-character-set combination in a group with a wildcard (the branch .. or parts inside a max_repeat listed above). You can put character classes inside a character set to avoid this.

Also note, that you can use the r'' raw string notation to avoid all the escaped backslashes.

The following works:

re.sub(r'\.[\W\.]*[o0][\Wo0]*', '*', '..........................................')

because it compiles to:

>>> re.compile(r'\.[\W\.]*[o0][\Wo0]*', re.DEBUG)
literal 46
max_repeat 0 65535
  in
    category category_not_word
    literal 46
in
  literal 111
  literal 48
max_repeat 0 65535
  in
    category category_not_word
    literal 111
    literal 48

Note that now the branches are gone.

来源：https://stackoverflow.com/questions/12014991/python-regular-expression-why-does-this-not-work

标签

python

regex

substitution