Python re.sub() weirdness

Question

I'm very new to Python, in fact this is my first script.

I'm struggling with Python's regular expressions. Specifically re.sub()

I have the following code:

variableTest = "192"
test = re.sub(r'(\$\{\d{1,2}\:)example.com(\})', r'\1' + variableTest + r'\2', searchString, re.M )

With this I'm trying to match something like host": "${9:example.com}" within searchString and replace example.com with a server name or IP address.

If variableTest contains an IP, it fails. I get the following error: sre_constants.error: invalid group reference

I've tested it with variableTest equal to "127.0.0.1", "1", "192", "192.168". "127.0.0.1" works while the rest doesn't. If I prepend the others with a letter it also works.

variableTest is a string - verified with type(variableTest)

I'm totally lost as to why this is.

If I remove r'\1' in the replacement string it also works. r'\1' will containt ${\d}:, with \d a number between 1 and 999.

Any help will be greatly appreciated!

Answer 1

The problem is that putting an IP in variableTest will result in a replacement string like this:

r'\18.8.8.8\2'

As you can see, the first group reference is to group 18, not group 1. Hence, re complains about the invalid group reference.

In this case, you want to use the \g<n> syntax instead:

r'\g<1>' + variableTest + r'\g<2>'

which produces e.g. r'\g<1>8.8.8.8\g<2>'.