parse

$ ruby-parse -E -e 'def a(*) end
a = 1
a(:a)
a :a'
def a(*) end
^~~ kDEF "def"                                  expr_fname   [0 <= cond] [0 <= cmdarg]
def a(*) end
    ^ tIDENTIFIER "a"                           expr_endfn   [0 <= cond] [0 <= cmdarg]
def a(*) end
     ^ tLPAREN2 "("                             expr_beg     [0 <= cond] [0 <= cmdarg]
def a(*) end
      ^ tSTAR "*"                               expr_beg     [0 <= cond] [0 <= cmdarg]
def a(*) end
       ^ tRPAREN ")"                            expr_end     [0 <= cond] [0 <= cmdarg]
  >>> STATE SET <<<                             expr_value   [0 <= cond] [0 <= cmdarg]
def a(*) end
         ^~~ kEND "end"                         expr_end     [0 <= cond] [0 <= cmdarg]
def a(*) end
            ^ tNL nil                           line_begin   [0 <= cond] [0 <= cmdarg]
a = 1
^ tIDENTIFIER "a"                               expr_cmdarg  [0 <= cond] [0 <= cmdarg]
a = 1
  ^ tEQL "="                                    expr_beg     [0 <= cond] [0 <= cmdarg]
a = 1
    ^ tINTEGER 1                                expr_end     [0 <= cond] [0 <= cmdarg]
a = 1
     ^ tNL nil                                  line_begin   [0 <= cond] [0 <= cmdarg]
a(:a)
^ tIDENTIFIER "a"                               expr_end     [0 <= cond] [0 <= cmdarg]
a(:a)
 ^ tLPAREN2 "("                                 expr_beg     [0 <= cond] [0 <= cmdarg]
a(:a)
  ^~ tSYMBOL "a"                                expr_end     [0 <= cond] [0 <= cmdarg]
a(:a)
    ^ tRPAREN ")"                               expr_end     [0 <= cond] [0 <= cmdarg]
a(:a)
     ^ tNL nil                                  line_begin   [0 <= cond] [0 <= cmdarg]
a :a
^ tIDENTIFIER "a"                               expr_end     [0 <= cond] [0 <= cmdarg]
a :a
  ^ tCOLON ":"                                  expr_beg     [0 <= cond] [0 <= cmdarg]
(fragment:0):4:3: error: unexpected token tCOLON
a :a
  ^

shadow.rb

RUBY_VERSION     #=> "2.1.1"
RUBY_PATCHLEVEL  #=> 76

def a(*) end
a :a  #=> nil

a = 1
a :a
SyntaxError: (irb):4: syntax error, unexpected ':', expecting end-of-input
	from /Users/Larry/.rbenv/versions/2.1.1/bin/irb:11:in `<main>'

a(:a)  #=> nil
a 'a'  #=> nil