Backreferences

A regular expression, in the strict sense, has no memory. It walks the subject and decides; it cannot say "the same thing I just saw". A backreference is the feature that breaks that rule, and this chapter is about what it buys and what it costs.

Four lessons, in reading order:

Lesson	The question
What a backreference is	\1 matches the text group 1 captured — spelled in Python, Perl, JavaScript, Go, Rust, grep and ABAP.
Forward references	The same \1, written before its group. Legal in Perl, PCRE2, ABAP, Java, Ruby and JavaScript; a compile error in Python. And the four names people confuse with it.
Backreferences in the replacement	The \1 on the right-hand side of a substitution, which is not a backreference and does not behave like one.
What a backreference costs	Why Go's RE2 and Rust's regex refuse the feature, measured rather than asserted.

If you are new to the whole area: the first lesson is self-contained. If you have used \1 for years and want the part that surprises people, start with forward references.

The one sentence the chapter keeps returning to: a backreference matches text, never a pattern. Everything else — why it makes matching exponential, why RE2 has none, why an unset group is a philosophical question — follows from that.