[Bro] Allocating Dynamic Bytestring Length

[TQ]

Hello There,

I have some data that is delimited by a comma, but the data length could
vary in between. For example:

1) A,B,C,D,E
2) AA,BBB,C,DDDDD,EE
3) AAA,BB,CCCCCCCC,DD,EEE

I was thinking of using bytestring inside the record and have it read until
each delimiter... something like...

type My_Data = record {

field1 : bytestring & length=readuntil 0x2c;
field2 : bytestring & length=readuntil 0x2c;
field3 : bytestring & length=readuntil 0x2c;
field4 : bytestring & length=readuntil 0x2c;
field1 : bytestring &restofdata;

};

Does this feature exist in Bro? I vaguely remember seeing a readuntil
feature for Bro, but can't pull up the exact info.

Thanks,