Replaces the first instance of match with replacement in s. match/replacement can be: char / char string / string pattern / (string or function of match). See also replace. The replacement is literal (i.e. none of its characters are treated specially) for all cases above except pattern / string. For pattern / string, $1, $2, etc. in the replacement string are substituted with the string that matched the corresponding parenthesized group in the pattern. If you wish your replacement string r to be used literally, use (re-quote-replacement r) as the replacement argument. See also documentation for java.util.regex.Matcher's appendReplacement method. Example: (clojure.string/replace-first "swap first two words" #"(\w+)(\s+)(\w+)" "$3$2$1") -> "first swap two words"