Synopsis:
   $index(<characters> <text>)
   $rindex(<characters> <text>)

Technical:
   These functions are the equivalent of the BSD library functions index()
   and rindex() (and their ANSI equivalents, strchr() and strrchr()).  They
   return the index to the first character in the given text that appears
   in the list of characters.

   The $index() function searches the text left to right, and $rindex()
   searches right to left.  The index, however, always counts from left to
   right, starting at 0 (zero).  The first word passed to the function is
   assumed to be the list of characters to look for; to insert a space, it
   must be escaped with a \, unless it is the first character.  To insert a
   backslash, it must also be escaped.

Practical:
   These functions are generally most useful in argument processing.  For
   instance, they can be used to parse a list of command-line switches
   passed to an alias.

Returns:
     -1   no matches found
   > -1   index to character

Examples:
   $index(abc hello there bob)            returns 12
   $index(xyz hello there bob)            returns -1
   $rindex(abc hello there bob)           returns 14
   $rindex(xyz hello there bob)           returns -1

Other Notes:
   Keep in mind that a failed search returns -1, not 0.  This has already
   been stated, but it is misused often enough that this reminder is
   warranted.