How to use eval or other method in calculated fields to extract values into a new field?

[reach2tushar]

Hi,

I am thinking of using the Calculated Fields option to extract one field.
I have following values in a field name "YOURFIELD"
Test_X
TestA_Y
TestBC_Z_all
I want to extract the characters before "_" in a new field "MYFIELD". The result will be:
Test
TestA
TestBC

Can please help me to extract this result in calculated fields using an EVAL function or any other method?

[Ayn]

... | rex field=YOURFIELD "(?<MYFIELD>.+?)_"

[Ayn]

Yes, you can.

[reach2tushar]

Thanks for the reply Ayn. Can we use RegEx in Calculated fields?