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JavaScript

// This example first splits the string using the digit 1. Then adds
// the digit 2 to the patter to split at 12.


var str = 'stackoverflow 11 Javascript 12 Regex Help',
    pattern=/1/g,
    newPattern,
    newString,
    previous;

// Write with original pattern as split
document.write(str.split(pattern).join('<br>'));

document.writeln('<br>--------------------------------------<br>');

// Extract old pattern and modifier if present
previous = pattern.toString().match(/\/(.*)\/(.*)/);

// Create new pattern
newPattern = new RegExp(previous[1] + '2', previous[2]);

// and split
newString=str.split(newPattern);

// Output with new split
document.write(newString.join('<br>'));
document.writeln('<br>--------------------------------------<br>' + newPattern.toString());