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.