Back reference into negative lookahead assertion¶
Back references can be used to refer to the result of a previously matched capture group. It is syntactically legal to refer from outside a negative lookahead assertion to a capture group nested inside that assertion, but since the regular expression can only match when the body of the negative lookahead assertion did not match, such a back reference always matches the empty string. This probably indicates a mistake.
Remove the back reference if it is useless, or fix the regular expression to make sure the reference refers to the intended capture group.
In the following example, the back reference
\2 refers to the capture group
(a+) inside the negative lookahead assertion
Useless back references like this can arise if a regular expression is updated inconsistently. In this example, for instance, the group
(?:d*) may initially have been capturing, so the back reference
\2 would have referred to it instead of the capture group inside the negative lookahead assertion. If this is the case, the group
(?:d*) should be made capturing again, that is, it should be replaced by
Note that referring to a capture group from within the same negative lookahead assertion is unproblematic.
Ecma International, ECMAScript Language Definition, 5.1 Edition, Section 18.104.22.168. ECMA, 2011.