really is an extended regular expression, please read
.Xr grep 1
manual page for more information about its format.
.Sh ENVIRONMENT
.Bl -tag -width ".Ev PAGER"
.It Ev PATH , MANPATH , MANLOCALES
Used in finding the
.Nm whatis
database files.
See
.Xr manpath 1 .
.It Ev PAGER
Command used to page the output.
The default is
.Ic pager -s .
.El
.Sh DIAGNOSTICS
The
.Nm