Javascript Regular Expression to match 5 or 9 digit zipcode

Add anchors: new RegExp("^\\d{5}(-\\d{4})?$"). This forces the regular expression engine to only accept a match, if it begins at the first character of the string (^) and ends at the end of the string ($) being matched.

Note, that there might be a typo in the regular expression you hav given in your question: the second \d is missing a backslash.

Change your regex to:

new RegExp("^\\d{5}(-\\d{4})?$")