Bug in awk

[omprakaashveroy . balasubramanian]

Hi,
Please see the attached text file. It has 5 fields delimited by commas.

The following command 
 awk ' BEGIN { FS="," } {a=index($4,"e"); printf 
"%s\n",substr($4,a-13,a+13) } ' active.txt 
should give me an output like shown below

8TM22N30OGK1QeAV717RFPJG1BG
BLG2MAHEFE0T0eBNV9FGBMUTQK9
C5FI84N0NAVACe5ASR0FJBRI70I
B3L18INGH1743e1OOPIPKIOJVSE
FVK6PJM0546E4eAD211EO7BC8KE

but it gives output like this with ~GM~M2 in addition to what is expected

8TM22N30OGK1QeAV717RFPJG1BG~GM~M2
BLG2MAHEFE0T0eBNV9FGBMUTQK9~GM~M2
C5FI84N0NAVACe5ASR0FJBRI70I~GM~M2
B3L18INGH1743e1OOPIPKIOJVSE~GM~M2
FVK6PJM0546E4eAD211EO7BC8KE~GM~M2

Please let us know why ?? 
NOTE : We are working on Linux 2.4.21-138-8SP3



Thanks & Best Regards
Omprakaashve Roy Balasubramanian
Tata Consultancy Services Limited
Mailto: address@hidden
Website: http://www.tcs.com
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active.txt
Description: Text document