          regsub ?-all? ?-nocase? exp string subSpec varName
               This command matches the regular expression exp against
               string using the rules described in REGULAR EXPRESSIONS
               above.   If there is no match, then the command returns
               0 and does nothing else.  If there is a match, then the
               command  returns  1  and  also  copies  string  to  the
               variable whose name is given by varName.  When  copying
               string,  the  portion  of  string  that  matched exp is
               replaced with subSpec.  If subSpec contains a ``&''  or
               ``\0'',  then  it  is replaced in the substitution with
               the portion of string that  matched  exp.   If  subSpec
               contains  a ``\n'', where n is a digit between 1 and 9,
               then it  is  replaced  in  the  substitution  with  the
               portion  of  string that matched the n-th parenthesized
               subexpression of exp.  Additional  backslashes  may  be
               used  in  subSpec  to prevent special interpretation of
               ``&'' or ``\0'' or ``\n'' or  backslash.   The  use  of
               backslashes in subSpec tends to interact badly with the
               Tcl parser's use  of  backslashes,  so  it's  generally
               safest  to  enclose  subSpec  in  braces if it includes
               backslashes.  If the -all argument is  specified,  then
               all  ranges  in  string  that  match  exp are found and
               substitution is performed for  each  of  these  ranges;
               otherwise  only  the  first matching range is found and
               substituted.  If -all  is  specified,  then  ``&''  and
               ``\n''  sequences  are  handled  for  each substitution
               using the information from the corresponding match.  If
               the  -nocase  argument  is  specified,  then upper-case
               characters in string are converted to lower-case before
               matching against exp;  however, substitutions specified
               by subSpec use the original unconverted form of string.
               The  -all  and  -nocase  arguments  must  be  specified
               exactly:  no abbreviations are permitted.
