#FROM DATABASE TEXT
$line = "Her house is on 34th Mt. Whitney St. near St. Mt. Helens St.";

#FROM INCOMING FORM INPUTS
$query = "(St\.\s)(Mt\.\s)(?=Helens)";
$substitution = "Mount ${1}";    # USER MAY HAVE ENTERED "$1"

#FOR RETURNED HTML HIGHLIGHT OF CHANGES
$start = qq|<span class="highlight">|;
$end = "</span>";

return "Regex containing code disallowed." if $query =~ m[\(\?\??\{];
return "Regex containing code disallowed." if $substitution =~ m[\(\?\??\{];


my $replace = sub {
  my $evaluate = sub {
      return eval($1);
  };
    my $val = $substitution;
    $val =~ s/(\$\{\d+\})/$evaluate->()/eg;
    $val = "$start$val$end";
    return eval($val);
};

    $line =~ s/$query/$replace->()/eg;

#EXPECTED TEXT AFTER SUBSTITUTION
$line = "Her house is on 34th Mt. Whitney St. near Mount St. Helens St.";