You can reset the start of a regex in Ruby
The \K escape is the “Match-Reset Anchor”, which resets the start of the reported match, and skip any previously consumed characters.
I wanted to write a regex which replaced characters in the middle of a matched string – I was looking for a fixed prefix, which I wanted to leave unchanged, then modify the characters after that. Initially I tried to capture the prefix in a group, then use that group into the replacement string, but I was struggling to get it working.
Then I discovered the Match-Reset Anchor \K, which excludes anything preceding it from the result. If I add that after the fixed prefix, I don’t need to include it in the replacement.
Here’s a simple example:
"abc".gsub(/ab\Kc/, "C") # => "abC"
If the explanation is unclear, looking at the website plugin where I used this may be more meaningful.