U

u (regular expression) modifier, Regexp Literals

u (Unicode) escape, Double-quoted string literals, Double-quoted string literals

UCS (Universal Character Set), Multibyte characters in Ruby 1.9

UDPServer class, Datagrams

unary +/–, Unary + and –, Memoizing Functions

UnboundMethod class, Unbound Method Objects

undef keyword, Undefining Methods

undefine_finalizer method (ObjectSpace), ObjectSpace and GC

undefining methods, Undefining Methods

underscore (_), Integer Literals, Constant References

constants, Constant References

integer literals, using, Integer Literals

ungetc method, Reading bytes and characters

unhandled exceptions, Threads and unhandled exceptions

Unicode, Unicode Codepoint Constants with const_missing

const_missing method and, Unicode Codepoint Constants with const_missing

Unicode characters, Unicode characters in identifiers, Double-quoted string literals, Unicode escapes

escapes, Double-quoted string literals, Unicode escapes

uninitialized variables, Uninitialized Variables

union method, Adding and deleting set elements

Universal Character Set (UCS), Multibyte characters in Ruby 1.9

unless keyword, Operators, unless

unreachable objects, Object Lifetime

until keyword, Operators, while and until, while and until As Modifiers

modifiers, as, while and until As Modifiers

while loops and, while and until

untrace_var method (Kernel), Tracing

upto method, Blocks and Iterators, Numeric Iterators

upto method (Integer), Iterators and Enumerable Objects

UTF-8 encoding, Unicode characters in identifiers, Specifying Program Encoding, Unicode escapes, Regular expression encoding