Regular Expression for a string

[John Machin]

Teja wrote:
> 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

Based on the examples you give later, you don't mean "optional" (0 or 1
occurrences), you mean 0 or more occurrences.
Assuming "alphabet" means "English alphabetic":

r"[a-zA-Z][a-zA-Z0-9_]*(\[[a-z0-9]\])*"

HTH,
John