Question

I want to match the longest sequence that is repeating at least once

Having:

T_send_ack-new_amend_pending-cancel-replace_replaced_cancel_pending-cancel-replace_replaced

the result should be: pending-cancel-replace_replaced

Answer 1

Try this

(.+)(?=.*\1)

See it here on Regexr

This will match any character sequence with at least one character, that is repeated later on in the string.

You would need to store your matches and decide which one is the longest afterwards.

This solution requires your regex flavour to support backreferences and lookaheads.

it will match any character sequence with at least one character .+ and store it in the group 1 because of the brackets around it. The next step is the positive lookahead (?=.*\1), it will be true if the captured sequence occurs at a later point again in the string.