strings.ReplaceRE

{{ $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

You can write and test your regular expression using regex101.com. Be sure to select the Go flavor before you begin.