oniguruma recursive.re

(?x)
(?<one>             # start of bracket 1
\(                  # match an opening angle bracket
    (?:               
        [^\(\)]++   # one or more non angle brackets, non backtracking
          |                  
        \g<one>     # recurse to bracket 1
    )*                 
\)                  # match a closing angle bracket
)                   # end of bracket 1