How can I just extract one underbar-separated field from a filename?

Question

I have a list of file names like this:

REG_2016120200hourly_d01_20161202_00_00_00.nc

Of this name I would like to extract and put in a variable:

Answer 1

timestep="${filename:34:2}"

2 is length.

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From man bash:

${parameter:offset:length}: Substring Expansion. Expands to up to length characters of the value of parameter starting at the character specified by offset. [...]