Detect if a string is a data URL. Doesn't try to parse it or determine validity, just a quick check if a string appears to be a data URL. See http://jsfiddle.net/bgrins/aZWTB/ for a demo.

detectDataURL.js

// Detecting data URLs
// data URI - MDN https://developer.mozilla.org/en-US/docs/data_URIs
// The "data" URL scheme: http://tools.ietf.org/html/rfc2397
// Valid URL Characters: http://tools.ietf.org/html/rfc2396#section2

function isDataURL(s) {
    return !!s.match(isDataURL.regex);
}
isDataURL.regex = /^\s*data:([a-z]+\/[a-z]+(;[a-z\-]+\=[a-z\-]+)?)?(;base64)?,[a-z0-9\!\$\&\'\,\(\)\*\+\,\;\=\-\.\_\~\:\@\/\?\%\s]*\s*$/i;

test.js

var yes = [
    "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0lEQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC",
    "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIBAMAAAA2IaO4AAAAFVBMVEXk5OTn5+ft7e319fX29vb5+fn///++GUmVAAAALUlEQVQIHWNICnYLZnALTgpmMGYIFWYIZTA2ZFAzTTFlSDFVMwVyQhmAwsYMAKDaBy0axX/iAAAAAElFTkSuQmCC",
    "   data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIBAMAAAA2IaO4AAAAFVBMVEXk5OTn5+ft7e319fX29vb5+fn///++GUmVAAAALUlEQVQIHWNICnYLZnALTgpmMGYIFWYIZTA2ZFAzTTFlSDFVMwVyQhmAwsYMAKDaBy0axX/iAAAAAElFTkSuQmCC   ",
    " data:,Hello%2C%20World!",
    " data:,Hello World!",
    " data:text/plain;base64,SGVsbG8sIFdvcmxkIQ%3D%3D",
    " data:text/html,%3Ch1%3EHello%2C%20World!%3C%2Fh1%3E",
    "data:,A%20brief%20note",
    "data:text/html;charset=US-ASCII,%3Ch1%3EHello!%3C%2Fh1%3E"
];

var no = [
    "dataxbase64",
    "data:HelloWorld",
    "data:text/html;charset=,%3Ch1%3EHello!%3C%2Fh1%3E",
    "data:text/html;charset,%3Ch1%3EHello!%3C%2Fh1%3E", "data:base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0lEQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC",
    "",
    "http://wikipedia.org",
    "base64",
    "iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0lEQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC"
];

var log = document.createElement("pre");
document.body.appendChild(log);

function printError(msg) {
   var message = document.createElement("span");
   message.style.color = "red";
   message.textContent = msg + "\n";
    
   log.appendChild(message);
}

function printSuccess(msg) {
   var message = document.createElement("span");
   message.style.color = "green";
   message.textContent = msg + "\n";

   log.appendChild(message);
}

yes.forEach(function(s) {
    if (!isDataURL(s)) {
        printError("Expected yes, got no: " + s);
    }
    else {        
        printSuccess("Expected yes, got yes: " + s);
    }
});

no.forEach(function(s) {
    if (isDataURL(s)) {
        printError("Expected no, got yes: " + s);
    }
    else {
        printSuccess("Expected no, got no: " + s);
    }
});

uri-data-syntax.md

Syntax

dataurl    := "data:" [ mediatype ] [ ";base64" ] "," data
mediatype  := [ type "/" subtype ] *( ";" parameter )
data       := *urlchar
parameter  := attribute "=" value

where "urlchar" is imported from [RFC2396], and "type", "subtype", "attribute" and "value" are the corresponding tokens from [RFC2045], represented using URL escaped encoding of [RFC2396] as necessary.

Attribute values in [RFC2045] are allowed to be either represented as tokens or as quoted strings. However, within a "data" URL, the "quoted-string" representation would be awkward, since the quote mark is itself not a valid urlchar. For this reason, parameter values should use the URL Escaped encoding instead of quoted string if the parameter values contain any "tspecial".

The ";base64" extension is distinguishable from a content-type parameter by the fact that it doesn't have a following "=" sign.

thanks

Thanks, it works great, here is a demo