Dynamic rules

            {my %braces = qw /{ } < > ( ) [ ]/;
                $braces {'\\'} = '\\\\';}

start:      string /^\Z/ {"That's a string!"}
          | <error: That's not a string!>

string:     /["']/ <skip: ""> /(?:[^$item[1]\\]+|\\.)*/ "$item[1]"
          | /qq?/ /./ <skip: ""> {$braces {$item [2]} || $item [2]}
                              /(?:[^$item[4]\\]+|\\.)*/ /[$item[4]]/

In [1] := q [foo bar]
Out [1] = That's a string!
In [2] := q {foo bar}
Out [2] = That's a string!
In [3] := q =foo bar=
Out [3] = That's a string!
In [4] := "foo bar"
Out [4] = That's a string!
In [5] := q [foo \]bar]
Out [5] = That's a string!
In [6] := q \foo bar\
Out [6] = That's a string!
In [7] := q !foo bar!
Out [7] = That's a string!
In [8] := 'foo \'bar ""'
Out [8] = That's a string!
In [9] := 'foo bar"

       ERROR (line 1):  That's not a string!
Out [9] = <<UNDEF>>
In [10] :=