こんな感じのコードを最近みました。

puts 'hello world!'      .gsub('hello', 'good evening')

いやいや！スペース空きすぎでしょ、こんなんエラー起こしているでしょー

なんて思ってたら、なんか動いていたんですね。

上のコードだと、結果はこうなります。

puts 'hello world!'      .gsub('hello', 'good evening')
# => good evening world!

なんだこれは、意味がわからん。僕の知っているRubyじゃない。ということで、字句解析と構文解析をしてみました。

require 'ripper'
require 'pp'

code =<<EOF
puts 'hello world!'      .gsub('hello', 'good evenning')
EOF

puts '=====lexer====='
pp Ripper.lex(code)
puts '=====parser====='
pp Ripper.sexp(code)

結果はこちら

=====lexer=====
[[[1, 0], :on_ident, "puts"],
 [[1, 4], :on_sp, " "],
 [[1, 5], :on_tstring_beg, "'"],
 [[1, 6], :on_tstring_content, "hello world!"],
 [[1, 18], :on_tstring_end, "'"],
 [[1, 19], :on_sp, "      "],
 [[1, 25], :on_period, "."],
 [[1, 26], :on_ident, "gsub"],
 [[1, 30], :on_lparen, "("],
 [[1, 31], :on_tstring_beg, "'"],
 [[1, 32], :on_tstring_content, "hello"],
 [[1, 37], :on_tstring_end, "'"],
 [[1, 38], :on_comma, ","],
 [[1, 39], :on_sp, " "],
 [[1, 40], :on_tstring_beg, "'"],
 [[1, 41], :on_tstring_content, "good evenning"],
 [[1, 54], :on_tstring_end, "'"],
 [[1, 55], :on_rparen, ")"],
 [[1, 56], :on_nl, "\n"]]
=====parser=====
[:program,
 [[:command,
   [:@ident, "puts", [1, 0]],
   [:args_add_block,
    [[:method_add_arg,
      [:call,
       [:string_literal,
        [:string_content, [:@tstring_content, "hello world!", [1, 6]]]],
       :".",
       [:@ident, "gsub", [1, 26]]],
      [:arg_paren,
       [:args_add_block,
        [[:string_literal,
          [:string_content, [:@tstring_content, "hello", [1, 32]]]],
         [:string_literal,
          [:string_content, [:@tstring_content, "good evenning", [1, 41]]]]],
        false]]]],
    false]]]]

重要なのは5つ目のon_spのところではないでしょうか。

字句解析の結果、Rubyは'hello world!'と.の間のスペースをon_spというトークンとしてひとまとめにしています。
そしてその後、構文解析の結果でon_spが何かに置き換わった様子もありません。
(parserの8行目で'hello world!'は:string_contentとして、
gsubは10行目で:@ident(identifier)として解析されている。)

つまり、Rubyではスペースを字句解析の段階でトークンとトークンを分けるためだけに使っているのではないかと思いました。(間違っていたらごめんなさい。)
なので、結果的に、スペースがあってもなくても構文解析の結果は、、、

require 'ripper'
require 'pp'

code =<<EOF
puts 'hello world!'.gsub('hello', 'good evenning')
EOF

puts '=====lexer====='
pp Ripper.lex(code)
puts '=====parser====='
pp Ripper.sexp(code)

=====lexer=====
[[[1, 0], :on_ident, "puts"],
 [[1, 4], :on_sp, " "],
 [[1, 5], :on_tstring_beg, "'"],
 [[1, 6], :on_tstring_content, "hello world!"],
 [[1, 18], :on_tstring_end, "'"],
 [[1, 19], :on_period, "."],
 [[1, 20], :on_ident, "gsub"],
 [[1, 24], :on_lparen, "("],
 [[1, 25], :on_tstring_beg, "'"],
 [[1, 26], :on_tstring_content, "hello"],
 [[1, 31], :on_tstring_end, "'"],
 [[1, 32], :on_comma, ","],
 [[1, 33], :on_sp, " "],
 [[1, 34], :on_tstring_beg, "'"],
 [[1, 35], :on_tstring_content, "good evenning"],
 [[1, 48], :on_tstring_end, "'"],
 [[1, 49], :on_rparen, ")"],
 [[1, 50], :on_nl, "\n"]]
=====parser=====
[:program,
 [[:command,
   [:@ident, "puts", [1, 0]],
   [:args_add_block,
    [[:method_add_arg,
      [:call,
       [:string_literal,
        [:string_content, [:@tstring_content, "hello world!", [1, 6]]]],
       :".",
       [:@ident, "gsub", [1, 20]]],
      [:arg_paren,
       [:args_add_block,
        [[:string_literal,
          [:string_content, [:@tstring_content, "hello", [1, 26]]]],
         [:string_literal,
          [:string_content, [:@tstring_content, "good evenning", [1, 35]]]]],
        false]]]],
    false]]]]

同じですね。

[スペースめっちゃあり]

[:program,
 [[:command,
   [:@ident, "puts", [1, 0]],
   [:args_add_block,
    [[:method_add_arg,
      [:call,
       [:string_literal,
        [:string_content, [:@tstring_content, "hello world!", [1, 6]]]],
       :".",
       [:@ident, "gsub", [1, 26]]],
      [:arg_paren,
       [:args_add_block,
        [[:string_literal,
          [:string_content, [:@tstring_content, "hello", [1, 32]]]],
         [:string_literal,
          [:string_content, [:@tstring_content, "good evenning", [1, 41]]]]],
        false]]]],
    false]]]]

[スペースなし]

[:program,
 [[:command,
   [:@ident, "puts", [1, 0]],
   [:args_add_block,
    [[:method_add_arg,
      [:call,
       [:string_literal,
        [:string_content, [:@tstring_content, "hello world!", [1, 6]]]],
       :".",
       [:@ident, "gsub", [1, 20]]],
      [:arg_paren,
       [:args_add_block,
        [[:string_literal,
          [:string_content, [:@tstring_content, "hello", [1, 26]]]],
         [:string_literal,
          [:string_content, [:@tstring_content, "good evenning", [1, 35]]]]],
        false]]]],
    false]]]]

なので、先ほどの

puts 'hello world!'      .gsub('hello', 'good evening')

は正常に動いていたのだと考えられます。

実験

他にも、スペースが必要な場面も解析してみました。
以下のコードを解析すると

[1,2].each do |e|
end

=====lexer=====
[[[1, 0], :on_lbracket, "["],
 [[1, 1], :on_int, "1"],
 [[1, 2], :on_comma, ","],
 [[1, 3], :on_int, "2"],
 [[1, 4], :on_rbracket, "]"],
 [[1, 5], :on_period, "."],
 [[1, 6], :on_ident, "each"],
 [[1, 10], :on_sp, " "],
 [[1, 11], :on_kw, "do"],
 [[1, 13], :on_sp, " "],
 [[1, 14], :on_op, "|"],
 [[1, 15], :on_ident, "e"],
 [[1, 16], :on_op, "|"],
 [[1, 17], :on_ignored_nl, "\n"],
 [[2, 0], :on_kw, "end"],
 [[2, 3], :on_nl, "\n"]]
=====parser=====
[:program,
 [[:method_add_block,
   [:call,
    [:array, [[:@int, "1", [1, 1]], [:@int, "2", [1, 3]]]],
    :".",
    [:@ident, "each", [1, 6]]],
   [:do_block,
    [:block_var,
     [:params, [[:@ident, "e", [1, 15]]], nil, nil, nil, nil, nil, nil],
     false],
    [[:void_stmt]]]]]]

lexerの７つ目の要素でeachをしっかりトークンにして、ASTにも変換できていますが、

eachとdoの間にスペースを入れないと。。。

=====lexer=====
[[[1, 0], :on_lbracket, "["],
 [[1, 1], :on_int, "1"],
 [[1, 2], :on_comma, ","],
 [[1, 3], :on_int, "2"],
 [[1, 4], :on_rbracket, "]"],
 [[1, 5], :on_period, "."],
 [[1, 6], :on_ident, "eachdo"],
 [[1, 12], :on_sp, " "],
 [[1, 13], :on_op, "|"],
 [[1, 14], :on_ident, "e"],
 [[1, 15], :on_op, "|"],
 [[1, 16], :on_ignored_nl, "\n"],
 [[2, 0], :on_kw, "end"],
 [[2, 3], :on_nl, "\n"]]
=====parser=====
nil

eachとdoがくっついてeachdoになり、構文解析で失敗しているようです。