RegExp in Javascript - Negating chars

[BomberMan2K]

Hi,
I want to validate a field for SPSS - i.e I want it to be only alphanumeric include under-scope, but it can't start with a number or an under-scope (and can't end with it).
I use the following regex: /^[a-zA-Z0-9_]+$/
But it only validates for alphanumerics. no negation like i want.

Please help - Want changes are needed to negate the start and end with under-scope and start with number.

Thank you for any help,
Roman.

 

[tsuji]

[tt]/^([a-zA-Z]{1,2}|[a-zA-Z][a-zA-Z0-9_]+[a-zA-Z])$/[/tt]