regular expression

Hi,
I want to implement a regular expression

 ?Any number of numerals followed by an underscore followed by any number of alphabets" ex:123_abc

 

 I aM TESTINg by inputting  at tempURL parameter How Can I do That I want that what i inpu shuld match this

Any number of numerals followed by an underscore followed by any number of alphabets

 

I have following code

var Url="[0-9]{1,}[_]{1,1}[A-Za-z]{1,}"
var tempURL="12A_3_abc";
var matchURL=temp ...