Use calculations on sub-expressions

Given the following file:

abc3
abc4
bcd1
bcd3

I'd like to find the line containing bcd1 having a gap (bcd2) between it and the next line.

Searching like this:

/^\(...\)\(\d\)\n\1\=\2+2
/^\(...\)\(\d\)\n\1\=submatch(2)+2
/^\(...\)\(\d\)\n\1\=eval(submatch(2)+2)

seemingly doesn't work. I wonder if there is any way to accomplish this task.

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