Ruby regex- does gsub store what it matches?

If i use

.gsub(/matchthisregex/,"replace_with_this")

does gsub store what it matches with the regex somewhere? I'd like to use what it matches in my replacement string. For example something like

"replace_with_" + matchedregexstring + "this"

in my above example where the matchedregexstring would be the stored match from gsub? Sorry if that was confusing, I don't know how else to word that.