Regex that only matches on odd/even indices

Question

Is there a regex that matches a string only when it starts on an odd or an even index? My use case is a hex string in which I want to replace certain "bytes".

Now, when trying to match 20 (space), 20 in "7209" would be matched as well even though it consists of the bytes 72 and 09. I am restricted to the regex implementation of Notepad++ in this case, so I'm not able to check the match index as e.g. in Java.

My sample input looks like:

324F8D8A20561205231920

I set up a testing page here, the regex should only match the first and the last occurence of 20, since the one in the middle starts on an odd index.

Answer 1

You can use the following regex to match 20 at even positions inside a hex string:

20(?=(?:[\da-fA-F]{2})*$)

See demo

I assume the string has no spaces in this case.

In case you have spaces between the values (or any other symbols), this could be an alternative (with $1XX-like replacement string):

((?:.{2})*?)20

See another demo

Answer 2

This seems to work for evens:

rx <- "^(.{2})*(20)"

strings <- c("7209","2079","9720")

grepl(rx,strings) # [1] FALSE  TRUE  TRUE