Is there a regular expression to detect a valid regular expression?

Question

Is it possible to detect a valid regular expression with another regular expression? If so please give example code below.

Answer 1

Though it is perfectly possible to use a recursive regex as MizardX has posted, for this kind of things it is much more useful a parser. Regexes were originally intended to be used with regular languages, being recursive or having balancing groups is just a patch.

The language that defines valid regexes is actually a context free grammar, and you should use an appropriate parser for handling it. Here is an example for a university project for parsing simple regexes (without most constructs). It uses JavaCC. And yes, comments are in Spanish, though method names are pretty self-explanatory.

SKIP :
{
    " "
|   "\r"
|   "\t"
|   "\n"
}
TOKEN : 
{
    < DIGITO: ["0" - "9"] >
|   < MAYUSCULA: ["A" - "Z"] >
|   < MINUSCULA: ["a" - "z"] >
|   < LAMBDA: "LAMBDA" >
|   < VACIO: "VACIO" >
}

IRegularExpression Expression() :
{
    IRegularExpression r; 
}
{
    r=Alternation() { return r; }
}

// Matchea disyunciones: ER | ER
IRegularExpression Alternation() :
{
    IRegularExpression r1 = null, r2 = null; 
}
{
    r1=Concatenation() ( "|" r2=Alternation() )?
    { 
        if (r2 == null) {
            return r1;
        } else {
            return createAlternation(r1,r2);
        } 
    }
}

// Matchea concatenaciones: ER.ER
IRegularExpression Concatenation() :
{
    IRegularExpression r1 = null, r2 = null; 
}
{
    r1=Repetition() ( "." r2=Repetition() { r1 = createConcatenation(r1,r2); } )*
    { return r1; }
}

// Matchea repeticiones: ER*
IRegularExpression Repetition() :
{
    IRegularExpression r; 
}
{
    r=Atom() ( "*" { r = createRepetition(r); } )*
    { return r; }
}

// Matchea regex atomicas: (ER), Terminal, Vacio, Lambda
IRegularExpression Atom() :
{
    String t;
    IRegularExpression r;
}
{
    ( "(" r=Expression() ")" {return r;}) 
    | t=Terminal() { return createTerminal(t); }
    |  { return createLambda(); }
    |  { return createEmpty(); }
}

// Matchea un terminal (digito o minuscula) y devuelve su valor
String Terminal() :
{
    Token t;
}
{
    ( t= | t= ) { return t.image; }
}