Could even pass through Filter::cpp or cpp, but that might be overkill.

I guess you "just" have to reverse the regular expression given in perlfaq6 or perldoc -q comments:

\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"
+'\\]*)
[download]

Or maybe just make that one noncapturing and capture everything in between.

I started the work based on that regexp. Completely untested. Like that regexp, my code is for C (not C++), so some changes are required. Hopefully I got all the different types of escapes, but I didn't check any documentation.

sub handle_token {
   my ($type, $value, $pos) = @_;
   return if $type eq 'comment';
   return if $value !~ /byFoo/;
   print("Found byFoo in $type starting at byte $pos\n");
}


# ^ User code
# v Lexer code


my %str_escapes = (
   t => chr(0x09),
   n => chr(0x0A),
   r => chr(0x0D),
   ...
);


sub handle_char_escapes {
   my ($s, $pos) = @_;

   for ($s) {
      s/\\([a-wy-z])/
         if (exists($str_escapes{$1})) {
            $str_escapes{$1}
         } else {
            warn("Unrecognized escape sequence \"\\$1\"\n");
            $1
         }
      /eg;

      s/\([0-9]+)/
         die("Bad input: Octal escape sequence too big in string at po
+s $pos\n");
            if length($1) > 3 || oct($1) > 255;
         chr(oct($1))
      /eg;

      s/\x([0-9a-fA-F]+)/
         die("Bad input: Hex escape sequence too big in string at pos 
+$pos\n");
            if length($1) > 2 || oct("0x$1") > 255;
         chr(hex("0x$1"))
      /eg;

      s/\\(.)/$1/sg;
   }


   return $s;
}


sub handle_comment {
   my ($raw_comment, $pos) = @_;
   handle_token('comment', substr($raw_comment, 2, -2), $pos);
}


sub handle_string {
   my ($raw_string, $pos) = @_;
   handle_token(
      'string', 
      handle_char_escapes(substr($raw_string, 1, -1), $pos),
      $pos
   );
}


sub handle_char {
   my ($raw_char, $pos) = @_;
   handle_token(
      'char',
      handle_char_escapes(substr($raw_char, 1, -1), $pos),
      $pos
   );
}


sub handle_code {
   handle_token('comment', @_);
}


sub lex {
   for ($_[0]) {
      / \G ( \/\*[^*]*\*+(?:[^\/*][^*]*\*+)*\/ ) /xgc && do { handle_c
+omment("$1", $-[0]); redo };
      / \G ( "(?:\\.|[^"\\]*)"                 ) /xgc && do { handle_s
+tring ("$1", $-[0]); redo };
      / \G ( '(?:\\.|[^'\\])*'                 ) /xgc && do { handle_c
+har   ("$1", $-[0]); redo };
      / \G ( .[^/"'\\]*                        ) /xgc && do { handle_c
+ode   ("$1", $-[0]); redo };

      / \G \z / && last;
      die("Bad input\n");
   }
}


# ^ Lexer code
# v User code


lex($c_source);
[download]

...roboticus