How to find and replace nth occurrence of word in a sentence using python regular expression?

Question

Using python regular expression only, how to find and replace nth occurrence of word in a sentence? For example:

str = \'cat goose  mouse horse pig cat cow\'         


        

Answer 1

Create a repl function to pass into re.sub(). Except... the trick is to make it a class so you can track the call count.

class ReplWrapper(object):
    def __init__(self, replacement, occurrence):
        self.count = 0
        self.replacement = replacement
        self.occurrence = occurrence
    def repl(self, match):
        self.count += 1
        if self.occurrence == 0 or self.occurrence == self.count:
            return match.expand(self.replacement)
        else:
            try:
                return match.group(0)
            except IndexError:
                return match.group(0)

Then use it like this:

myrepl = ReplWrapper(r'Bull', 0) # replaces all instances in a string
new_str = re.sub(r'cat', myrepl.repl, str)

myrepl = ReplWrapper(r'Bull', 1) # replaces 1st instance in a string
new_str = re.sub(r'cat', myrepl.repl, str)

myrepl = ReplWrapper(r'Bull', 2) # replaces 2nd instance in a string
new_str = re.sub(r'cat', myrepl.repl, str)

I'm sure there is a more clever way to avoid using a class, but this seemed straight-forward enough to explain. Also, be sure to return match.expand() as just returning the replacement value is not technically correct of someone decides to use \1 type templates.