Variable substitution

I need to strip the forward slash out of a variable “DAHDI/g0”. I’ve read the devel docs on substitutions but am not well versed in regex. I’ve gotten “DAHDI” from the variable with this:

exten => s,n,Set(TRNK=$["${TRNK}" : “([a-zA-Z][a-zA-Z][a-zA-Z][a-zA-Z][a-zA-Z])”])

but anything after that is lost. If I could use sed that would be great but I’m lost with this. Help!!