File Processing

i am having the input file as
below

123456789: xxxxx12xxxxxxxxxxxxxxxxxx a_cnt[1]
123456789: xxxxxxxxxxxxxxxxxxxxxxx a_cnt[2]
123456789: a_cnt[3] xxxxaq1wsxxxxxxxxxxxx12xxxxxxxxxx
123456789: xxxxxxxxxxxxasxxxx a_cnt[4]

i need the numbers in the backets of a_cnt[**]

O/p required as below
1
2
3
4

thanks in advance..

what have you tried so far? please post the logic.

thanks guys.. i have done it by separating the similar lines.