## Backreferencing

Backreferencing is used to match same text again. Backreferences match the same text as previously matched by a capturing group. Let's look at an example:

Note: `\/` is escaped `/`character, check it out in the appendix.

The first captured group is (`\w+`), now we can use this group again by using a backreference (`\1`) at the closing tag, which matches the same text as in captured group `\w+`.

You can backreference any captured group by using `\group_no`.

Let's have two more examples:

### Backreferencing and character class

Backreferencing can not be used in character class. Let's see an example:

### Backreferencing and Quantifiers

When we are using a backreference for an expression with quantifiers, then we have to be careful. Let's observe it:

Note that `(\d)+` and `(\d+)` both are different. So, what will happen for `(\d)+ -- \1` expression and same text above?

Can you observe something?

For `(\d)+ -- \1` expression and `123 -- 3` string, first time 1 was stored in \1, then 2 was stored in \1 and at last 3 was stored. So, it will show a match if and only if the last character before `--` is exactly same as the character after `--`.

Problems:

1. Match any palindrome string of length 6, having only lowercase letters. Answer: `([a-z])([a-z])([a-z])\3\2\1`

2. RegEx: `(\w+)oo\1le`
   Text: `google, doodle jump, ggooggle, ssoosle`

   Answer:

Note: For group numbers more than 9, there is a syntax difference.