Golang中的正则表达“不是常量”错误

I'm trying to write a regex in Golang to verify that a string only has alphanumerics, periods, and underscores. However, I'm running into an error that I haven't seen before and have been unsuccessful at Googling.

我正在尝试在Golang中编写一个正则表达式来验证字符串是否只有字母数字,句点和下划线。但是,我遇到了一个我以前没有见过的错误,并且在Googling上没有成功。

Here's the regex:

这是正则表达式:

pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

Here is the error:

这是错误:

const initializer regexp.MustCompile("^[A-Za-z0-9_\\.]+") is not a constant

What does "not a constant" mean and how do I fix this?
Thanks.

什么“不是常数”是什么意思,我该如何解决这个问题?谢谢。

3 个解决方案

#1

This typically happens when you're trying to assign to a package-level variable that has a type that can't be constant (like for example, Regexp). Only basic types likes int, string, etc. can be constant. See here for more details.

当您尝试分配具有不能为常量的类型的包级变量时(例如,Regexp),通常会发生这种情况。只有基本类型如int,string等可以是常量。有关详细信息,请参见此处

Example:

pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)
// which translates to:
const pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

You have to declare it as a var for it to work:

你必须将它声明为var才能工作:

var pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

In addition, I usually put a note to say that the variable is treated as a constant:

另外,我通常会说明变量被视为常量:

var /* const */ pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

#2

The error is pretty clear. If you are trying to do this globally...

错误非常清楚。如果你想在全球范围内这样做......

Don't do:

const pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

Instead do:

var pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

Or if you really want the pattern in a constant:

或者如果你真的希望模式保持不变:

const pattern = `^[A-Za-z0-9_\.]+`
var alphaNum = regexp.MustCompile(pattern)

#3

In Go declaration context, assignment with a simple = creates a constant, not a variable. (Outside of a declaration, it's an assignment to a variable that must already exist.)

在Go声明上下文中,使用simple =赋值会创建常量,而不是变量。 (在声明之外,它是对必须已存在的变量的赋值。)

But a constant initialization has to include only constants - not calls like regexp.MustCompile() - so pattern can't be a constant in this case, even if you don't plan on changing its value later. (In fact, even if you could somehow initialize it without calling anything, a Regexp can't be a constant in Go; only basic types can be.)

但是,常量初始化必须只包含常量 - 而不是像regexp.MustCompile()这样的调用 - 因此,在这种情况下,模式不能是常量,即使您不打算稍后更改其值。 (事实上,即使你能以某种方式初始化它而不调用任何东西,Regexp也不能成为Go中的常量;只有基本类型才可以。)

That means you need to make it a variable by either putting it inside a var statement or declaring it inside a function with := instead of =:

这意味着您需要通过将其置于var语句中或在函数内使用:=而不是=声明它来使变量成为变量:

var (
   pattern = ...
)

var pattern = ...

func something() {
   pattern := ...
}

#1