condition in regular expression

Hi,

I would like to match a regular expression, which can take two forms:

aaa1(-1)&abcd

or

bbb2&aefdg

More specifically,

• the expression starts with word characters (e.g. aaa1 or bbb2)
• it is optionally followed by a parenthesis, then a minus sign, then a closing parenthesis
• it is followed by the ampersand (&)
• it is followed by word characters and then

I would like to capture those expressions and I wrote the following code, which does not work well

expr='\<(?<before>\w+)\>(\(-)?(?<digit>\d+)?(\))?&(?<after>\w+)';

regexp('vvv&mp  abvg(-5)&ads  abvg-5&ads',expr,'names')

It does not work well because the third expression (abvg-5&ads) should not be a match. That is, the digits should be matched only if they are around parentheses.

I thought that maybe using some form of condition

(?(cond)expr1|expr2)

would help but I was not successful in implementing it. Maybe that is the way to go, maybe there is another way, I don't know.

Any suggestions?

Thanks