Java RegEx back slash Number [duplicate]

Question

This question already has answers here:

What's the meaning of a number after a backslash in a regular expression? (2 answers)

Closed 9 years ago.

What does it mean to have a \number in a regex in java.

Let's say I have something like \1 or \2. What does this mean and how is it used? An example would be really helpful.

Thanks

Answer 1

Backreferences match the same text as previously matched by a capturing group. Suppose you want to match a pair of opening and closing HTML tags, and the text in between. By putting the opening tag into a backreference, we can reuse the name of the tag for the closing tag. Here's how:

<([A-Z][A-Z0-9]*)\b[^>]*>.*?</\1>

This regex contains only one pair of parentheses, which capture the string matched by

[A-Z][A-Z0-9]*

The backreference \1 (backslash one) references the first capturing group. \1 matches the exact same text that was matched by the first capturing group. The / before it is a literal character. It is simply the forward slash in the closing HTML tag that we are trying to match.

For more details and examples check: http://www.regular-expressions.info/backref.html

Answer 2

\ usually is used at the start of the construction of a match. It also represents an escape character.