[Regular Expressions] AWK not working according to specification given in manual

[Kalidas Yeturu]

Hi

echo "1S7Q
   abckd" | awk '/[:alnum:]{4}/{print $0}'

Does NOT OUTPUT 1S7Q as expected

Details:
awk --version
GNU Awk 3.1.5

uname -a
Linux adenine.physics.iisc.ernet.in 2.6.21-1.3194.fc7 #1 SMP Wed May 23
22:47:07 EDT 2007 x86_64 x86_64 x86_64 GNU/Linux

Please help.

Thanks in advance
-- 
With Regards
Kalidas Y
http://ssl.serc.iisc.ernet.in/~kalidas