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.