<regex>: Some escape sequences are mishandled

[muellerj2]

There are a number of escape sequences that the parser mistakenly accepts or miscompiles.

ECMAScript

• Backreferences with leading zero digits (e.g., \01 for capture group 1) should be rejected. [ECMA-262 3rd ed., Section 15.10.2.11 "DecimalEscape"]
• \00 and more zero digits should be rejected and not be interpreted as an escape for NUL. Only \0 is a valid escape sequence for NUL. [ECMA-262 3rd ed., Section 15.10.2.11 "DecimalEscape"]
• When a custom traits implementation defines a new character class "z", [\z] matches the characters in this class and not the character z. (Meanwhile, \z without brackets matches the character z and not the characters in the class "z".) [ECMA-262 3rd ed., Sections 15.10.1 "Patterns" and 15.10.2.12 "CharacterClassEscape"]
• [\b] should match U+0008 BACKSPACE, not b. [ECMA-262 3rd ed., Section 15.10.2.19 "ClassEscape"]

awk

See Section "Regular expressions" in the awk specification.

• Octal escape sequences are not parsed correctly in square-bracket character class definitions. (E.g., [\040] should match U+0020 SPACE.)
• Similarly, [\"] and [\/] match backslashes as well even though they shouldn't.
• While the awk specification says that using unspecified escape sequences results in undefined behavior, I think we should reject them. (I believe we should handle this differently from ECMAScript mode, where unrecognized escape sequences just yield the escaped character.)