{{ $s := "a-b--c---d" }}
{{ replaceRE `(-{2,})` "-" $s }} → a-b-c-d
Limit the number of replacements using the LIMIT argument:
{{ $s := "a-b--c---d" }}
{{ replaceRE `(-{2,})` "-" $s 1 }} → a-b-c---d
Use $1, $2, etc. within the replacement string to insert the content of each capturing group within the regular expression:
{{ $s := "http://gohugo.io/docs" }}
{{ replaceRE "^https?://([^/]+).*" "$1" $s }} → gohugo.io