What does $1, $2, etc. mean in Regular Expressions?

Time and time again I see $1 and $2 being used in code. What does it mean? Can you please include examples?

When you create a regular expression you have the option of capturing portions of the match and saving them as placeholders. They are numbered starting at $1.

For instance:

/A(\d+)B(\d+)C/

This will capture from A90B3C the values 90 and 3. If you need to group things but don't want to capture them, use the (?:...) version instead of (...).

The numbers start from left to right in the order the brackets are open. That means:

/A((\d+)B)(\d+)C/

Matching against the same string will capture 90B, 90 and 3.

This is esp. useful for Replacement String Syntax (i.e. Format Strings) Goes good for Cases/Case Foldings for Find & Replaces. To reference a capture, use $n where n is the capture register number. Using $0 means the entire match. Example : Find: (<a.*?>)(.*?)(</a>) Replace: $1\u$2\e$3