how to select of this line "data[new][formhidden_field1]" here it is "formhidden_field1"?

how to select of this line "data[new][formhidden_field1]" here it is "formhidden_field1"?

how to select of this line "data[new][formhidden_field1]" this "formhidden_field1"?

I want to select the text "formhidden_field1" in the variable.