ruby - 为什么只有有限数量的正则表达式捕获存储在 `global_variables` 中？

Question

如果我用包含十个捕获的正则表达式进行匹配:

/(o)(t)(th)(f)(fi)(s)(se)(e)(n)(t)/.match("otthffisseent")

然后，对于 $10，我得到:

$10 # => "t"

但 global_variables 中缺少它。我得到(在 irb session 中):

[:$;, :$-F, :$@, :$!, :$SAFE, :$~, :$&, :$`, :$', :$+, :$=, :$KCODE, :$-K, :$,,
 :$/, :$-0, :$\, :$_, :$stdin, :$stdout, :$stderr, :$>, :$<, :$., :$FILENAME,
 :$-i, :$*, :$?, :$$, :$:, :$-I, :$LOAD_PATH, :$", :$LOADED_FEATURES,
 :$VERBOSE, :$-v, :$-w, :$-W, :$DEBUG, :$-d, :$0, :$PROGRAM_NAME, :$-p, :$-l,
 :$-a, :$binding, :$1, :$2, :$3, :$4, :$5, :$6, :$7, :$8, :$9]

这里只列出前九个:

$1, :$2, :$3, :$4, :$5, :$6, :$7, :$8, :$9

这也得到了证实:

global_variables.include?(:$10) # => false

$10 存储在哪里，为什么不存储在 global_variables 中？

Answer 1

Ruby 似乎在解析器级别处理 $1、$2 等:

ruby --dump parsetree_with_comment -e '$100'

输出:

###########################################################
## Do NOT use this node dump for any purpose other than  ##
## debug and research.  Compatibility is not guaranteed. ##
###########################################################

# @ NODE_SCOPE (line: 1)
# | # new scope
# | # format: [nd_tbl]: local table, [nd_args]: arguments, [nd_body]: body
# +- nd_tbl (local table): (empty)
# +- nd_args (arguments):
# |   (null node)
# +- nd_body (body):
#     @ NODE_NTH_REF (line: 1)
#     | # nth special variable reference
#     | # format: $[nd_nth]
#     | # example: $1, $2, ..
#     +- nd_nth (variable): $100

顺便说一句，maximum number of capture groups是 32,767，您可以通过 $n 访问所有内容:

/#{'()' * 32768}/       #=> RegexpError: too many capture groups are specified

/#{'()' * 32767}/ =~ '' #=> 0
defined? $32767         #=> "global-variable"
$32767                  #=> ""