Documentation / Regular Expressions / Backreferences \1
Backreferences \1
Backreferences match the previously captured subexpression. Subexpressions are marked with parentheses and numbered from 1 to 9. Use \1 to refer to the leftmost subexpression, \2 to refer to the second subexpression, and so on.
| (\d+) \1 | Find two equal numbers with space between them (e.g., 123 123) |
| \<(\w+) \1\> | Find repeating words (e.g., the the) |
| <a href=("|')http://[^'"]+\1> | An external link (in single or double quotes) |