Disallow backreferences that will always be replaced with the empty string.
configuration in plugin:clean-regex/recommended: "error"
Backreferences that will always be replaced with the empty string serve no function and can be removed.
The easiest case of this is if the references capturing group does not consume
any characters (e.g. (\b)a\1). Since the capturing group can only capture the
empty string, the backreference is essentially useless.
Examples of valid code for this rule:
/(a?)b\1/
/(\b|a)+b\1/Examples of invalid code for this rule:
/(|)a\1/
/(\b)a\1/
/(^|(?=.))a\1/If a backreference cannot be reached from the position of the referenced capturing group without resetting the captured text, then the backreference will always be replaced with the empty string.
If a group (capturing or non-capturing) is entered by the Javascript regex
engine, the captured text of all capturing groups inside the group is reset. So
even though a backreference might be reachable for the position of its
referenced capturing group, the captured text might have been reset. An example
of this is (?:\1(a)){2}. The \1 is reachable after (a) in the second
iteration but the captured text of (a) is reset by their parent non-capturing
group before \1 can matched (in the second iteration). This means that the
Javascript regex /^(?:\1(a)){2}$/ only accepts the string aa but not aaa.
(Note: The regex engines of other programming languages may behave differently.
I.e. with Python's re module, the regex ^(?:\1(a)){2}$ will accept the string
aaa but not aa.)
Backreferences that appear before their referenced capturing group (e.g.
\1(a)) will always be replaced with the empty string.
Please note that before depends on the current matching direction. RegExps are
usually matched from left to right but inside lookbehind groups, text is matched
from right to left. I.e. the pattern (?<=\1(a))b will match all bs preceded
by two as.
Examples of valid code for this rule:
/(a)?(?:a|\1)/Examples of invalid code for this rule:
/\1(a)/
/(a\1)/
/(a)|\1/
/(?:(a)|\1)+/ // the \1 can be reached but not without resetting the captured text