I'm trying to use a regular expression to validate a multi-line entry of 4 letters followed by 6 numbers. Following that requirement, the user may enter a comma followed by a number, or hit <Enter> to enter a second required value:
abcd000001
abcd000002,1
abcd000003,16
The comma is optional, but if present must be followed by a number.

For the "if comma then match number" I tried this, but I get a syntax error.
  var re = new RegExp(/[A-Za-z]{4}\d{6}\,?(?(1)\d+)/);

Everything through the comma works:
  var re = new RegExp(/[A-Za-z]{4}\d{6}\,?/);
but the if-then part causes the error.

Try this...
HTH,

Chris

That works, however "abcd000001,a" causes this if statement to eval to true.

  var re = new RegExp(/[A-Za-z]{4}\d{6}(,\d+)?/);
  if (document.Form1.txt.value.match(re)) {
    return true;
  } else {
    return false;
  }

You need to add criteria for the start & end of the string as well...

Perfect. Thanks.