An assertion such as (? =ab(c_BAR_de)) is not permitted, because its single top-level branch can match two different lengths, but it is acceptable if rewritten to use two top-level branches: (? =abc_BAR_abde) The implementation of lookbehind assertions is, for each alternative, to temporarily move the current position backAn assertion such as (? =ab(c_BAR_de)) is not permitted, because its single top-level branch can match two different lengths, but it is acceptable if rewritten to use two top-level branches: (? =abc_BAR_abde) The implementation of lookbehind assertions is, for each alternative, to temporarily move the current position back by the fixed width and then try to match.
Pour la rendre acceptable, il faut écrire (? =abc_BAR_abde)