Regular Expression for a string

[Teja]

HI all,

I need to write a regular experssion for a string which satisfies the
following a criteria :

1) it should start with an alphabet
2) it can contain alphabets/digits/_ from second character
3) it can contain "[a-z]" or "[0-9]" at the end. but this is optional

 can any one please help me out.... pls....


Teja.P