try using [_a-zA-Z][_a-zA-Z0-9] rather than the expanded form. i didn't track it down to be sure, but plan 9 awk seem to have a limit 34 characters inside square brackets. a rather odd number
cpu% echo hello | awk '/^[abcdefghijklmnopqrstuvwxyzABCDEFGH].+/ {
line = $0 ; print line }'
cpu% echo hello | awk '/^[abcdefghijklmnopqrstuvwxyzABCDEFG].+/ { line
= $0 ; print line }'
hello
