Weird syntax error with parens

Question

Other than hashes as last argument, you can drop parenthesis in Ruby on method invocations and get consistent results (you still have to mind the priority).

However, I encountered an example where this is not the case:

''.split(/ ./) # => []
''.split /./   # => []
''.split / ./  # !> SyntaxError: unexpected '.'

Is this a bug/regression (I tested it with 2.1.2 -> 2.4.1 Rubys)?

Are there other generic cases where dropping the parens doesn't work as expected?

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Reported it, lets see.

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Update: The ticket was rejected a bit ambiguously. It's not clear if it's a bug or not, but it won't get fixed and using %r{} in these situations was suggested. The cause is indeed that the opening slash is interpreted as division.

Answer 1

In addition to thesecretmaster's answer, let's take a look inside the Ruby parser:

require 'ripper'
require 'pp'

pp Ripper.lex("''.split /./")
# [[[1,  0], :on_tstring_beg,     "'"    ],
#  [[1,  1], :on_tstring_end,     "'"    ],
#  [[1,  2], :on_period,          "."    ],
#  [[1,  3], :on_ident,           "split"],
#  [[1,  8], :on_sp,              " "    ],
#  [[1,  9], :on_regexp_beg,      "/"    ],
#  [[1, 10], :on_tstring_content, "."    ],
#  [[1, 11], :on_regexp_end,      "/"    ]]

Adding a space makes Ruby recognize the / characters as division operators:

pp Ripper.lex("''.split / ./")
# [[[1,  0], :on_tstring_beg, "'"    ],
#  [[1,  1], :on_tstring_end, "'"    ],
#  [[1,  2], :on_period,      "."    ],
#  [[1,  3], :on_ident,       "split"],
#  [[1,  8], :on_sp,          " "    ],
#  [[1,  9], :on_op,          "/"    ],
#  [[1, 10], :on_sp,          " "    ],
#  [[1, 11], :on_period,      "."    ],
#  [[1, 12], :on_op,          "/"    ]]

Are there other generic cases where dropping the parens doesn't work as expected?

Contrived example:

def foo(i = 1)
  10 * i
end

foo(- 2) #=> -20
foo - 2  #=> 8

Another one:

b = 2
def a(arg)
  arg
end

a *b    #=> 2

a = 5

a *b    #=> 10

The first a *b is interpreted as a(*b), whereas the second becomes a * b. Adding parentheses forces Ruby to invoke the method:

a(*b)   #=> 2

Answer 2

While this certainly seems like an issue, it isn't a very significant one because \s in regex will catch spaces. For example:

''.split(/ ./) # => []
''.split /./   # => []
''.split / ./  # !> SyntaxError: unexpected '.'
''.split /\s./ # => []

Or if you want to match only a space you just have to propperly escape it:

''.split /\ ./ # => []

Ruby should parse the arguments the same inside or outside parens, so I'd file a bug, but it's not very urgent and the ruby people may be happy simply leaving it.

Actually, @Stefan mentioned in comments to the questions that it's probably the parser reading it as devision, which is the most likely explanation, so it's not even a bug. But it's one of those fun little ruby quirks.

@Stefan also added that you can create an unambiguous regex literal with the %r{ .} syntax.