Question

How can I form a regular expression that match the unique numbers that repeat in a repeating decimals?

Currently my regular expressions is the following.

var re = /(?:[^\.]+\.\d*)(\d+)+(?:\1)$/;

Example:

// Pass
deepEqual( func(1/111), [ "0.009009009009009009", "009" ] );

// Fails, since func(11/111) returns [ "0.099099099099099", "9" ]
deepEqual( func(11/111), [ "0.099099099099099", "099" ] );

Live demo here: http://jsfiddle.net/9dGsw/

Here’s my code.

// Goal: Find the pattern within repeating decimals.
// Problem from: Ratio.js <https://github.com/LarryBattle/Ratio.js>

var func = function( val ){
    var re = /(?:[^\.]+\.\d*)(\d+)+(?:\1)$/;
    var match = re.exec( val );
    if( !match ){
        val = (val||"").toString().replace( /\d$/, '' );
        match = re.exec( val );
    }
    return match;
};
test("find repeating decimals.", function() {
    deepEqual( func(1), null );
    deepEqual( func(1/10), null );
    deepEqual( func(1/111), [ "0.009009009009009009", "009" ] );

    // This test case fails...
    deepEqual( func(11/111), [ "0.099099099099099", "099" ], 
        "What's wrong with re in func()?" );

    deepEqual( func(100/111), [ "0.9009009009009009", "009"] );
    deepEqual( func(1/3), [ "0.3333333333333333", "3"]);
});

Answer 1

Ok. I somewhat solved my own problem by taking Joel’s advice.

The problem was that the regular expression section, (\d+)+(?:\1)$, was matching the pattern closest to the end of the string, which made it return “9”, instead of “099” for the string “0.099099099099099”.

The way I overcame this problem was by setting the match length to 2 or greater, like so.

(\d{2,})+(?:\1)$,

and filtering the result with /^(\d+)(?:\1)$/, incase that a pattern is stuck inside a pattern.

Here’s the code that passes all my test cases.

Live Demo: http://jsfiddle.net/9dGsw/1/

var func = function( val ){
    val = (val || "").toString();
    var RE_PatternInRepeatDec = /(?:[^\.]+\.\d*)(\d{2,})+(?:\1)$/, 
        RE_RepeatingNums = /^(\d+)(?:\1)$/,
        match = RE_PatternInRepeatDec.exec( val );

    if( !match ){
        // Try again but take off last digit incase of precision error.
        val = val.replace( /\d$/, '' );
        match = RE_PatternInRepeatDec.exec( val );
    }
    if( match && 1 < match.length ){
        // Reset the match[1] if there is a pattern inside the matched pattern.
       match[1] = RE_RepeatingNums.test(match[1]) ? RE_RepeatingNums.exec(match[1])[1] : match[1];
    }
    return match;
};

Thank you for everyone that helped.