my( $pattern )= @_;
    my $error;
    my %helper= (
        '\w' => '[_[:alnum:]]',
        '\W' => '[^_[:alnum:]]',
        '\s' => '[[:space:]]',
        '\S' => '[^[:space:]]',
        '\d' => '[0-9]',
        '\D' => '[^0-9]',
        '\n' => "\n",
        '\t' => "\t",
        '\b' => '[[:<:]]',
        '\B' => '[[:>:]]',
        '\A' => '^',
        '\z' => '$',
        '\Z' => "\n?\$",
        '^'  => "(^|\n)",
        '$'  => "(\n|\$)",
    );
    my $prev= '';
    my @replace;
    my @mult= ( 0 );
    while(  $pattern =~ m<      # Tokenize the potential regex
          \G                    # Don't let it skip bits
            (                   # Return what we find
              \\x..             # A hexadecimal character
            | \\[012][0-7]{0,2} # An octal character
            | \\.               # A boring escaped character
            | [^\[\{]           # Not '[' nor '{' so treat as a token   #}
            | \{(?=\D)          # Literal but unescaped '{'             #}
            | \{\d+,?\d*}       # Bounded repetitions
            | \[                # '[' starts a character class
                \^?             #  '^' can go in front of the literal ']'
                \]?             #   ']' at start is taken literally
                (?:             #     Inside, there can be some subtokens
                  [^\]]         #       Not '[' so isn't a subtoken
                | \[(?=[^.=:])  #       '[' but not '[.', '[=', nor '[:'
                | \[            #       Must be '[' of '[.', '[=', or '[:'
                    [^\]]*      #       Anything but the closing ']'
                  \]            #       ']' closes out subtoken
                )*              #     Any number of subtokens
              \]                # ']' closes out the character class
            | (.)               # Found something invalid (sets $2)
            )
        >xsg
    ) {
        my $token= $1;
        if(  defined $2  ) {
            $error= "Your regex appears to be invalid."
              . " I understood '"
              . $q->escapeHTML(
                  substr( $pattern, 0, pos($pattern)-1 )
                ) . "' but not '"
              . $q->escapeHTML(
                  substr( $pattern, pos($pattern)-1 )
                ) . "'.";
            last;
        }
        if(  ')' eq $prev  ) {
            if(    0 < $mult[-1]
              and      $token =~ /^[+*]/
                   ||  $token =~ /^{./                  #}
            ) {
                $error= "We don't allow a () that contains"
                    . " +, *, or {} to have +, *, or {} applied to it"
                    . " (to prevent excess server load).";
                last;
            }
            my $mult= pop @mult;
            $mult[-1] += $mult;
        }
        if(  '(' eq $token  ) {
            push @mult, 0;
        } elsif(  $token =~ /^[+*]/  ||  $token =~ /^{./  ) {       #}
            $mult[-1]++;
        }
        if(  exists $helper{$token}  ) {
            push @replace, pos($pattern), length($token), $helper{$token};
        } elsif(  $token =~ /^\\x([0-9a-fA-F][0-9a-fA-F])\z/  ) {
            push @replace, pos($pattern), length($token), chr(hex($1));
        } elsif(  $token =~ /^\\([012][0-7]{0,2})\z/  ) {
            push @replace, pos($pattern), length($token), chr(oct($1));
        } elsif(  $token =~ /^\\\w\z/  ) {
            $error= "Illegal escape ($token) in regex.";
            last;
        }
        $prev= $token;
    }
    if(  6 < $mult[0]  ) {
        $error= "Your regex had $mult[0] instances of +, *, or {}."
            . " We don't allow more than 6 instances"
            . " (to prevent excess server load).";
    }
    if(  $error  ) {
        return "\\9$error";
    }
    while(  @replace  ) {
        my( $pos, $len, $str )= splice @replace, -3;
        substr( $pattern, $pos-$len, $len )= $str;
    }
    return $pattern;