regular expression that reference a match from earlier part of expression
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I'm looking for a regular expression that will identify a sequence in which an integer in the text specifies the number of trailing letters at the end of the expression.  This specific example applies to identifying insertions and deletions in genetic data in the pileup format.

For example: 

If the text I am searching is: 

'AtT+3ACGTTT-1AaTTa'

I need to match the insertions and deletions, which in this case are +3ACG and -1A. The integer (n) portion can be any integer larger than 1, and I must capture the n trailing characters.

I can match a single insertion or deletion with [+-]?[0-9]+[ACGTNacgtn], but I can't figure out how to grab the exact number of trailing ACGTN's specified by the integer.

I apologize if there is an obvious answer here, I have been searching for hours. Thanks!