How to edit my eval statement to convert numerical values from a field into MB?

[asarran]

I have a field [B] that consists of some numbers and strings.

10 gb
20 gb 
30 gb 

I would like to implement a eval statement that would convert the numerical values into MB.

B=*"GB"| eval GB=*/1024 | table GB 

I'm not sure what is incorrect about this statement?

[lweber]

if you have a 'string' field, first get the numeric value (i am using the rex command here)
then multiply the extracted value (field: numericB) with 1024 to get the MB...

| rex field=B "\D?(?<numericB>\d*)\D?" 
| eval MB=numericB*1024
| table MB