如何評估孤立的正則表達式

Question

我到處都看到這個片段，它有效！ 為什么？

while gets
    print if /start/../end/
end

ruby如何評估/start/沒有Lvalue？ 我希望我們首先必須將“獲取”的值存儲在某個地方然后再進行

gets_result =~ /start/.. gets_result =~ /end/

那么為什么代碼片段有效呢？

讓我澄清一下。

ruby如何知道將正則表達式與gets進行比較在上面的代碼片段中，我沒有指定ruby將reg exp與之比較gets但是它只知道。 問題是怎么樣的？

Answer 1

Kernel#gets不僅返回下一行，還將值賦給$_ 。

Kernel#print print $_如果沒有參數。

觸發器操作符（ /start/../end/ ）也在$_ 。

Answer 2

好問題。

記住：當在條件語句中使用Range運算符（ ..或... ）時，它會完全出乎意料：它不會創建Range對象。 相反，它充當“觸發器”操作員。

實際上是下面的代碼

while gets
    print if /start/../end/
end

默認是

while gets
    # gets_input I put just to make the code more expressive
    # actually the input taken using gets method applied here implicitly.
    print if  /start/ =~ gets_input .. /end/ =~ gets_input
end

讓我來證明你。 我接受了Ruby Tracer類的幫助。

trace = TracePoint.new do |tp|
  p [tp.lineno, tp.event, tp.defined_class,tp.method_id]
end
trace.enable do
  while gets
    # when you type start in your console, 11 will be output.
    print 11 if /start/../end/
  end
end

讓我運行這段代碼，向您展示我的上述代碼作為證明以及Ruby Flip-Flop功能：

(arup~>Ruby)$ ruby test.rb
test.rb:6: warning: regex literal in condition
test.rb:6: warning: regex literal in condition
[4, :b_call, nil, nil]
[5, :line, nil, nil]
[5, :c_call, Kernel, :gets]
[5, :c_call, ARGF.class, :gets]
end # I presses **end** here.
[5, :c_return, ARGF.class, :gets]
[5, :c_return, Kernel, :gets]
[6, :line, nil, nil]
# Regexp#=~ call begin happened for /end/ =~ gets_input
[6, :c_call, Regexp, :=~]
# Regexp#=~ call end happened for /end/ =~ gets_input
[6, :c_return, Regexp, :=~] 
[5, :c_call, Kernel, :gets]
[5, :c_call, ARGF.class, :gets]
start # I presses **start** here.
[5, :c_return, ARGF.class, :gets]
[5, :c_return, Kernel, :gets]
[6, :line, nil, nil]
# Regexp#=~ call begin happened for /start/ =~ gets_input
[6, :c_call, Regexp, :=~] 
# Regexp#=~ call end happened for /start/ =~ gets_input
[6, :c_return, Regexp, :=~]
# Regexp#=~ call begin happened for /end/ =~ gets_input
[6, :c_call, Regexp, :=~] 
# Regexp#=~ call end happened for /end/ =~ gets_input
[6, :c_return, Regexp, :=~] 
[6, :c_call, Kernel, :print]
[6, :c_call, IO, :write]
[6, :c_call, Fixnum, :to_s]
[6, :c_return, Fixnum, :to_s]
# As there is a match so **if** clause true, thus 11 printed
11[6, :c_return, IO, :write] 
[6, :c_return, Kernel, :print]
[5, :c_call, Kernel, :gets]
[5, :c_call, ARGF.class, :gets]
end  
[5, :c_return, ARGF.class, :gets]
[5, :c_return, Kernel, :gets]
[6, :line, nil, nil]
[6, :c_call, Regexp, :=~]
[6, :c_return, Regexp, :=~]
[6, :c_call, Kernel, :print]
[6, :c_call, IO, :write]
[6, :c_call, Fixnum, :to_s]
[6, :c_return, Fixnum, :to_s]
11[6, :c_return, IO, :write]
[6, :c_return, Kernel, :print]
[5, :c_call, Kernel, :gets]
[5, :c_call, ARGF.class, :gets]

Answer 3

在這種情況下， ..是觸發器操作符。 當給出'start'時，條件計算為true，並且繼續為true直到給出end 。