Question

Long story short, I have two regex patterns. One pattern matches things that I want to replace, and the other pattern matches a special case of those patterns that should not be replace. For a simple example, imagine that the first one is “\{.*\}” and the second one is “\{\{.*\}\}”. Then “{this}” should be replaced, but “{{this}}” should not. Is there an easy way to take a string and say “substitute all instances of the first string with “hello” so long as it is not matching the second string”?

In other words, is there a way to make a regex that is “matches the first string but not the second” easily without modifying the first string? I know that I could modify my first regex by hand to never match instances of the second, but as the first regex gets more complex, that gets very difficult.

Answer 1

You can give replace a function (reference)

But make sure the first regex contain the second one. This is just an example:

regex1 = re.compile('\{.*\}')
regex2 = re.compile('\{\{.*\}\}')

def replace(match):
    match = match.group(0)
    if regex2.match(match):
        return match
    return 'replacement'


regex1.sub(replace, data)