Back reference and forward reference in Regular Expressions

  • Back reference is the term used when a capturing group is referenced after we capture it. 
  • Forward reference is the term for referencing a group before capturing the text using a captured group. 
The forward reference refers to a capturing group that appears later in the regular expression pattern. Forward references are only useful if we use them inside a repeated group, since the regex engine will be able to resolve captured groups and populate the forward reference while repeating a match using quantifiers.

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