fixed_encoding?

        # Regexp.fixed_encoding?

(from ruby core)
---
    rxp.fixed_encoding?   -> true or false

---

Returns false if rxp is applicable to a string with any ASCII compatible
encoding. Returns true otherwise.

    r = /a/
    r.fixed_encoding?                               #=> false
    r =~ "\u{6666} a"                               #=> 2
    r =~ "\xa1\xa2 a".force_encoding("euc-jp")      #=> 2
    r =~ "abc".force_encoding("euc-jp")             #=> 0

    r = /a/u
    r.fixed_encoding?                               #=> true
    r.encoding                                      #=> #<Encoding:UTF-8>
    r =~ "\u{6666} a"                               #=> 2
    r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> Encoding::CompatibilityError
    r =~ "abc".force_encoding("euc-jp")             #=> 0

    r = /\u{6666}/
    r.fixed_encoding?                               #=> true
    r.encoding                                      #=> #<Encoding:UTF-8>
    r =~ "\u{6666} a"                               #=> 0
    r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> Encoding::CompatibilityError
    r =~ "abc".force_encoding("euc-jp")             #=> nil