Type: regular expression
A regular expression used by mutt to parse the GECOS field of a password entry when expanding the alias. By default the regular expression is set to "ˆ[ˆ,]*" which will return the string up to the first "," encountered. If the GECOS field contains a string like "lastname, firstname" then you should set the gecos_mask=".*".
This can be useful if you see the following behavior: you address a e-mail to user ID stevef whose full name is Steve Franklin. If mutt expands stevef to "Franklin" firstname.lastname@example.org then you should set the gecos_mask to a regular expression that will match the whole name so mutt will expand "Franklin" to "Franklin, Steve".