Back reference in a regular expression should be used after the referred group

  • Error
  • Medium
  • No tags

This rule applies when a back reference in a regular expression is used before the referred group matches.

A back reference like \1 is used to refer the input characters previously matched by a capture group. For example, /(foo|bar)\1/ will match foofoo or barbar but not foobar.

However, when a back reference is placed before the referred group, it will always match just the empty string. This is not likely to be a programmer's intent.

Noncompliant Code Example

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var re = /\1(foo|bar)/; // BAD_BACK_REFERENCE_IN_REGEXP alarm because the referred group '(foo|bar)` appears after the reference '\1'.

Compliant Code Example

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var re = /(foo|bar)\1/;


This rule was introduced in DeepScan 1.30.0.