Back references

Back references provide a convenient way of matching a repeated character or repeated tokens in the input text. By using back references, the regular expression engine can match the exact same text as previously matched by a capturing group.

The syntax of a back reference is a backslash followed by a capturing group number, as shown in the following example:

    \3 

The preceding example is a back reference of the third capturing group.

In Java regular expressions, there can be up to 99 back references, each number referencing a captured group number.

For example, if we need to match a two-digit number with the restriction that both digits must be the same, then we need to capture the first digit and then use a back reference ...

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