The indexof filter returns the index of a string in another string.

    indexof ("e" "hello)  1

If x and y are string filters, then indexof (x y) has value the index (zero-based) of the first occurrence of the string represented by x in the string represented by y. If either argument does not match, or if x does not occur in y, then the filter fails to match. This filter does not use regular expressions:

    indexof (y x+y)  3
    indexof (y x) // fails to match

index of group captures

To get the index of a group capture expression like \1 within the target string it matched to, use group capture index syntax, e.g. \-1 . Of course, you can still use indexof normally to find the location of \1 inside another string.
    \-0  3
    "mate: 23" ~~ "mate: (\d+)"
    \-0  0
    \-1  6
    \1 in "1234" // matches the position