Regex optional group

I am using this regex:

((?:[a-z][a-z]+))_(\d+)_((?:[a-z][a-z]+)\d+)_(\d{13})

to match strings like this:

SH_6208069141055_BC000388_20110412101855

separating into 4 groups:

SH
6208069141055
BC000388
20110412101855

Question: How do I make the first group optional, so that the resulting group is a empty string?
I want to get 4 groups in every case, when possible.

Input string for this case: (no underline after the first group)

6208069141055_BC000388_20110412101855

Making a non-capturing, zero to more matching group, you must append ?.

(?: ..... )?
^          ^____ optional
|____ group

You can easily simplify your regex to be this:

(?:([a-z]{2,})_)?(\d+)_([a-z]{2,}\d+)_(\d+)$
^              ^^
|--------------||
| first group  ||- quantifier for 0 or 1 time (essentially making it optional) 

I'm not sure whether the input string without the first group will have the underscore or not, but you can use the above regex if it's the whole string.

regex101 demo

As you can see, the matched group 1 in the second match is empty and starts at matched group 2.