Function: replace-regexp-in-string

Replace all matches for REGEXP with REP in STRING.

Return a new string containing the replacements.

Optional arguments FIXEDCASE, LITERAL and SUBEXP are like the
arguments with the same names of function `replace-match'. If START
is non-nil, start replacements at that index in STRING.

REP is either a string used as the NEWTEXT arg of `replace-match' or a
function. If it is a function, it is called with the actual text of each
match, and its value is used as the replacement text. When REP is called,
the match data are the result of matching REGEXP against a substring

To replace only the first match (if any), make REGEXP match up to \'
and replace a sub-expression, e.g.
(replace-regexp-in-string "\\(foo\\).*\\'" "bar" " foo foo" nil nil 1) => " bar foo"