Makes a  copy of the target sequence (the subject) with all matches 
of the  regular expression rgx (the pattern)  replaced  by  fmt
(the replacement).  The target sequence is either s or the character
sequence between first and last,  depending  on  the  version
used.

The resulting   sequence is returned as a string object on versions
1, 2, 3 and 4.  Versions 5 and 6 take an output iterator as  first
argument, which is used to store the resulting sequence.

An optional parameter,  flags, allows to specify options on how to
match and format the expression.