Ruby replace string with captured regex pattern

 "foobar".gsub(/(o+)/){|s|s+'ball'}
 #=> "fooballbar"

\1 in double quotes needs to be escaped. So you want either

"Z_sdsd: sdsd".gsub(/^(Z_.*): .*/, "\\1")

"Z_sdsd: sdsd".gsub(/^(Z_.*): .*/, '\1')

see the docs on gsub where it says "If it is a double-quoted string, both back-references must be preceded by an additional backslash."

That being said, if you just want the result of the match you can do:

"Z_sdsd: sdsd".scan(/^Z_.*(?=:)/)

"Z_sdsd: sdsd"[/^Z_.*(?=:)/]

Note that the (?=:) is a non-capturing group so that the : doesn't show up in your match.

Try '\1' for the replacement (single quotes are important, otherwise you need to escape the \):

"foo".gsub(/(o+)/, '\1\1\1')
#=> "foooooo"

But since you only seem to be interested in the capture group, note that you can index a string with a regex:

"foo"[/oo/]
#=> "oo"
"Z_123: foobar"[/^Z_.*(?=:)/]
#=> "Z_123"

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