Returns whether the given backreference will always be replaced with the empty string.
There are two reasons why a backreference might always be replaced with the empty string:
The referenced capturing group does not consume characters.
This is the trivial case. If the referenced capturing group never consumes any characters, then a backreference to
that group must be replaced with the empty string.
E.g. /(\b)a\1/
The backreference is not after the referenced capturing group.
A backreference can only be replaced with a non-empty string if the referenced capturing group has captured text
before the backreference is matched. There are multiple reasons why the capturing group might be unable to capture
text before a backreference to it is reached.
The capturing group might be in a different alternative. E.g. /(a)b|\1/.
The backreference might be inside the capturing group. E.g. /(a\1)/.
The backreference might be before the capturing group. E.g. /\1(a)/, /(?:\1(a))+/, /(?<=(a)\1)b/
Returns whether the given backreference will always be replaced with the empty string.
There are two reasons why a backreference might always be replaced with the empty string:
The referenced capturing group does not consume characters.
This is the trivial case. If the referenced capturing group never consumes any characters, then a backreference to that group must be replaced with the empty string.
E.g.
/(\b)a\1/The backreference is not after the referenced capturing group.
A backreference can only be replaced with a non-empty string if the referenced capturing group has captured text before the backreference is matched. There are multiple reasons why the capturing group might be unable to capture text before a backreference to it is reached.
/(a)b|\1/./(a\1)/./\1(a)/,/(?:\1(a))+/,/(?<=(a)\1)b/