在比较中使用正则表达式的结果(Using results of regular expression in a comparison)

在比较中使用正则表达式的结果(Using results of regular expression in a comparison)

我试图在比较操作中使用模式匹配的结果。 似乎无法让它发挥作用。

if语句在这里返回false：

if (('6kjkjd' =~ /(\d)/) > 5) { print "Why isn't this printing? 6 is greater than 5."; }

和这里一样：

my $start; if ((($start) = '6kjkj' =~ /(\d)/) > 5) { print "Why isn't this printing? 6 is greater than 5."; }

我已经尝试从比较操作的左侧删除parens，但这总是会产生一个真正的值：

my $start; if (($start) = '6kjkj' =~ /(\d)/ > 7) { print "This gets printed even though 6 is not greater than 7."; }

我意识到我可以把它分成两行，如下所示：

my ($start) = '6kjkj' =~ /(\d)/; if ($start > 5) { print "This works as expected."; }

尽管如此，我很想知道为什么前三个代码示例不能像您预期的那样工作。

I'm trying to use the result of pattern match in a comparison operation. Can't seem to get it to work.

The if statement returns false here:

if (('6kjkjd' =~ /(\d)/) > 5) { print "Why isn't this printing? 6 is greater than 5."; }

As well as here:

my $start; if ((($start) = '6kjkj' =~ /(\d)/) > 5) { print "Why isn't this printing? 6 is greater than 5."; }

I've tried removing the parens from the left side of comparison operation but that always results in a true value:

my $start; if (($start) = '6kjkj' =~ /(\d)/ > 7) { print "This gets printed even though 6 is not greater than 7."; }

I realize I could break this into two lines like so:

my ($start) = '6kjkj' =~ /(\d)/; if ($start > 5) { print "This works as expected."; }

Still, I'm curious to know why the first three code examples don't work as you might expect.

最满意答案

列表与标量上下文：

if (('6kjkjd' =~ /(\d)/)[0] > 5) { print "Why isn't this printing? 6 is greater than 5."; }

基本上，在if -condition的范围内，您处于标量上下文中。

当你写：

my $start; if ((($start) = '6kjkj' =~ /(\d)/) > 5)

$start的赋值发生了，但是list ($start)只有一个元素。

如果你坚持写一个复杂的表达式而不是把事情分成几部分，你可以利用逗号运算符：

if ((($start) = '6kjkj' =~ /(\d)/), $start > 5) { print "Why isn't this printing? 6 is greater than 5."; }

正如Sam Choukri在评论中指出的那样，

if (('6kjkj' =~ /(\d)/) and ($1 > 5)) {

比我最初想到的两种替代方案更安全，更简单。

List versus scalar context:

if (('6kjkjd' =~ /(\d)/)[0] > 5) { print "Why isn't this printing? 6 is greater than 5."; }

Basically, within the confines of an if-condition, you are in scalar context.

When you write:

my $start; if ((($start) = '6kjkj' =~ /(\d)/) > 5)

The assignment to $start happens, but the list ($start) has just one element.

If you insist on writing a complicated expression instead of breaking things into pieces, you can take advantage of the comma operator:

if ((($start) = '6kjkj' =~ /(\d)/), $start > 5) { print "Why isn't this printing? 6 is greater than 5."; }

As Sam Choukri points out in the comments,

if (('6kjkj' =~ /(\d)/) and ($1 > 5)) {

is safer, and simpler, than both alternatives that first came to my mind.