awk with two delimeters

Hi,

can anyone explain me below codes...i used the below one to use two delimeter(://) at a time..
but i am not able to understand the output options..
note: here there is no requirement ..only to understand myself about below command...

command:
awk -F "[://]" '{print $1}' inputfile

input file
//alt:haf:
//aal:as//skjlfk:

becuase some time i get output that i did not expect..let say $1 , $2,$3 etc...giving some different outputs..so it would be better if some one can explain two delimeter option in awk..
Thanks
Sha

The [] handles a class of single characters. You want something like:

awk -F ":|//" '{ print $2 }'

Note that $1 is actually the null-string before //.

You can specify 2 characters as a fieldseparator as follow:

awk -F "//|:" 

Regards