如何计算 Perl 中重叠的子字符串? [英] How can I count overlapping substrings in Perl?

问题描述

我需要实现一个程序来计算 perl 中字符串中子字符串的出现次数.我已经实现如下

sub countnmstr{$count = 0;$count++ 而 $_[0] =~/$_[1]/g;返回 $count;}$count = countnmstr("aaa","aa");打印 "$count
";

现在这是我通常会做的.但是，在上面的实现中，我想计算aaa"中aa"的出现次数.在这里，我得到的答案为 1，这似乎是合理的，但我也需要考虑重叠的情况.因此，上述情况应给出答案为 2，因为如果我们考虑重叠，则有两个aa".

谁能建议如何实现这样的功能??

解决方案

参见 ysth 的回答 ...我没有意识到该模式可以仅由零宽度断言组成，并且仍然可以用于此目的.

您可以按照其他人的建议使用 positive lookahead，并将函数写为:

sub countnmstr {我的 ($haystack, $needle) = @_;我的 ($first, $rest) = $needle =~/^(.)(.*)$/;返回标量 (() = $haystack =~/(Q$firstE(?=Q$restE))/g);}

您还可以使用 pos 来调整位置下一个搜索从:

#!/usr/bin/perl使用严格；使用警告；子计数nmstr {我的 ($haystack, $needle) = @_;我的 $adj = length($needle) - 1;die "搜索字符串不能为空！"如果 $adj <0;我的 $count = 0;而 ( $haystack =~/Q$needle/g ) {pos $haystack -= $adj;$count += 1;}返回 $count;}打印 countnmstr("aaa","aa"), "
";

输出:

<前>C:Temp> t2

i need to implement a program to count the occurrence of a substring in a string in perl. i have implemented it as follows

sub countnmstr
{
  $count =0;
  $count++ while $_[0] =~ /$_[1]/g;
  return $count;
}

$count = countnmstr("aaa","aa");

print "$count
";

now this is what i would normally do. however, in the implementation above i want to count occurrence of 'aa' in 'aaa'. here i get answer as 1 which seems reasonable but i need to consider the overlapping cases as well. hence the above case should give an answer as 2 since there are two 'aa's if we consider overlap.

can anyone suggest how to implement such a function??

解决方案

See ysth's answer ... I failed to realize that the pattern could consist solely of a zero width assertion and still work for this purpose.

You can use positive lookahead as suggested by others, and write the function as:

sub countnmstr {
    my ($haystack, $needle) = @_;
    my ($first, $rest) = $needle =~ /^(.)(.*)$/;
    return scalar (() = $haystack =~ /(Q$firstE(?=Q$restE))/g);
}

You can also use pos to adjust where the next search picks up from:

#!/usr/bin/perl

use strict; use warnings;

sub countnmstr {
    my ($haystack, $needle) = @_;
    my $adj = length($needle) - 1;
    die "Search string cannot be empty!" if $adj < 0;

    my $count = 0;
    while ( $haystack =~ /Q$needle/g ) {
        pos $haystack -= $adj;
        $count += 1;
    }
    return $count;
}

print countnmstr("aaa","aa"), "
";

Output:

C:Temp> t
2

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