arrays - Perl 检查数组中是否存在以单词开头的行，并将匹配的值返回给变量

Question

我检查了以下主题 Perl check a line contains at list one word of an array ，但我仍然困惑如何使其更适合我的情况。

我使用上面主题中的示例。

我有一个数组，名为@exampleWords:

my @exampleWords = ("balloon", "space", "monkey", "fruit" );

我有一行包含一个句子，例如:

my $line = "monkey space always unlimited";

如何检查 $line 是否以数组中的匹配单词开头，并将匹配的单词返回到变量中？

在上面的例子中，匹配的单词是“monkey”。

我目前的解决方案是:循环数组中的每个单词并检查 $line 是否以 $word 开头。

my $matchWord = "";
foreach my $word(@exampleWords) {
  if ($line =~ /^$word/) {
    $matchWord = $word;
    last;
  }
}

我仍在寻找更有效的解决方案..

谢谢...

Answer 1

原则上，您必须迭代可能的单词来匹配。但是，您还可以使用它们构建交替正则表达式模式，以便正则表达式引擎启动一次，这与每次迭代启动的循环不同。此外，现在迭代是通过高度优化的 C 代码进行的。

这些比较如何？让我们使用核心模块 Benchmark 对它们进行基准测试.

对于一个小数组，在其中间匹配(您的示例)

use warnings;
use strict;

use Benchmark qw( cmpthese );

my @ary = ("balloon", "space", "monkey", "fruit");
my $line = "monkey space always unlimited";

sub regex {
    my ($line, @ary) = @_;
    my $match; 
    my $re = join '|', map { quotemeta } @ary;
    if ($line =~ /^($re)/) {
        $match = $1;
    }   
    return $match;
}   

sub loop {
    my ($line, @ary) = @_;
    my $match; 
    foreach my $word (@ary) {
        if ($line =~ /^$word/) {  # see note at end
            $match = $word;
            last;
        }   
    }   
    return $match;
}   

cmpthese(-10, {
    regex => sub { regex ($line, @ary) },
    loop  => sub { loop  ($line, @ary) },
}); 

这会在一台装有 v5.16 的非常好的机器上和一台装有 v5.10 的旧机器上产生

          Rate  loop regexloop  222791/s    --  -70%regex 742962/s  233%    --

Thus regex is way more efficient.

For a 40 times larger array, matching around the middle

I build this array by @ary = qw(...) x 20, then add a word ('AHA'), then repeat 20 more times. I prepend that very word to the string, so that's what gets matched. I make the string much larger, too, even though this shouldn't matter for matching.

In this case the regex is even more convincing

         Rate  loop regexloop   9300/s    --  -82%regex 50873/s  447%    --

and yet more so with v5.10 on the older machine, with 574%.

On v5.27.2 the regex is faster by 1188%, so by a clean order of magnitude. But it is the rate of the loop that drops to only 6723/s, against the above 9330/s. So this only shows that the regex "startup" is more expensive in newer Perls, thus the loop falls further behind.

For the same large array, with the match word near its beginning

I move the match-word AHA in the array right past the original 4-word list

         Rate  loop regexloop  36710/s    --   -3%regex 37666/s    3%    --

So the match needs to happen very, very early so that the loop catches up with the regex. While this can happen often in specific use cases it cannot be expected in general, of course.

Note that the regex had far less work to do as well. Thus it's clear that the loop's problem is that it starts the regex engine anew for every iteration. Here it only had to do it a few times and the regex's advantage all but evaporated, even though it also matched much sooner.

---

As for programmer's efficiency, take your pick. There are yet other ways using higher level libraries so that you don't have to write the loop. For instance, using core List::Util

use List::Util qw(first);

my $match = first { $line =~ /^$_/ } @ary;

此基准与添加时的循环相同，但慢 10% 左右。

---

关于问题中使用的正则表达式的注释。

如果 $line 中的第一个单词是 puppy正则表达式 /^$word/将与 pup 匹配。这可能是有意的，也可能不是有意的(但可以将 flu 视为 fluent)，但如果不是，可以通过添加单词边界 anchor \b 来修复。 ,

$line =~ /^$word\b/

同样可以与交替模式一起使用，交替模式是为了模仿问题中的代码而编写的。因此添加单词边界 anchor ，即 /^($re)\b/ .

另一种方法是按单词长度对列表进行排序，sort { length $b <=> length $a } @ary ，每 Borodin的评论。这可能会以更复杂的方式影响问题，请考虑。