Noncapturing Parentheses ?:

if we use the parentheses in the RE, it will store the matched string to the memory. the first parentheses' string stored to $1 the second to $2 and so on. if we don't use the matched string, we can use ?: to ignore the matched one.

if (/(?:bronto)?saurus (steak|burger)/) {
    print "Fred wants a $1\n";
}

even if we add something to the RE, it will not change the group number

if (/(?:bronto)?saurus (?:BBQ )?(steak|burger)/) {
    print "Fred wants a $1\n";
}