Python – Locating the position of a regex match in a string?

Question

I’m currently using regular expressions to search through RSS feeds to find if certain words and phrases are mentioned, and would then like to extract the text on either side of the match as well. For example:

String = "This is an example sentence, it is for demonstration only"
re.search("is", String)

I’d like to know the position(s) of where the ‘is’ matches are found so that I can extract and output something like this:

1 match found: "This is an example sentence"

I know that it would be easy to do with splits, but I’d need to know what the index of first character of the match was in the string, which I don’t know how to find

Asked By: nb.

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Source

Answer 1

You could use .find("is"), it would return position of “is” in the string

or use .start() from re

>>> re.search("is", String).start()
2

Actually its match “is” from “This“

If you need to match per word, you should use b before and after “is”, b is the word boundary.

>>> re.search(r"bisb", String).start()
5
>>>

for more info about python regular expressions, docs here

Answered By: YOU

Answer 2

re.Match objects have a number of methods to help you with this:

>>> m = re.search("is", String)
>>> m.span()
(2, 4)
>>> m.start()
2
>>> m.end()
4

Answered By: SilentGhost

Answer 3

I don’t think this question has been completely answered yet because all of the answers only give single match examples. The OP’s question demonstrates the nuances of having 2 matches as well as a substring match which should not be reported because it is not a word/token.

To match multiple occurrences, one might do something like this:

iter = re.finditer(r"bisb", String)
indices = [m.start(0) for m in iter]

This would return a list of the two indices for the original string.

Answered By: demongolem

Python – Locating the position of a regex match in a string?

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