searching backwards in a string

[Mark McEahern]

[Paul Rubin]
> I have a string and want to search backwards for a regexp, starting at
> a given index, like if you search backwards in a text editor.  Is
> there a way to do that with re?  The docs aren't clear about what's
> supposed to happen if the ending position to the pat.search method is
> less than the starting position, but I tried it and it doesn't seem
> to do what I want.  Any advice appreciated--thanks.

Short answer:  Reverse the string first.

Sample:

    #! /usr/bin/env python
    # findBackwards.py

    import re

    def reverseWord(s):
        l = list(s)
        l.reverse()
        return ''.join(l)

    def findBackwards(s, pattern):
        """Return the location of the first match of pattern in s, searching
backwards."""
        r = reverseWord(s)
        m = re.search(pattern, r)
        if m is None:
            print "%s not found in %s." % (pattern, s)
            return
        # Express location relative to original orientation of word.
        start = len(s) - m.start()
        print "%s found backwards in %s at %d." % (pattern, s, start)

    s = "foobar"
    pattern = "o"
    findBackwards(s, pattern)

    pattern = "z"
    findBackwards(s, pattern)

Running:

    $ findBackwards.py
    o found backwards in foobar at 3.
    z not found in foobar.