(\d)\d\1
The \1 refers back to what was captured in the group enclosed by parentheses. If you have more than one Capturing Group, using \1, \2 as sequence referring.
(\d) matches the first digit and captures it\d matches the next digit but does not capture it because it is not enclosed in parentheses\1 references the captured digit707-827-7019Notice there’re four 7 in the number, so below we capture first 7, and refer it three times using Back References
(d)0\1\D\d\d\1\D\1\d\d\d
Date: 2025/04/28