On Mon, Feb 13, 2023 at 6:48 PM Pat Farrell <pat22...@gmail.com> wrote:
> This won't compile
>
> var ExtRegex =
> regexp.MustCompile("(M|m)(p|P)(3|4))|((F|f)(L|l)(A|a)(C|c))$")
>
> with a
> ./prog.go:10:18:
> regexp.MustCompile("((M|m)(p|P)(3|4))|((F|f)(L|l)(A|a)(C|c))$") (value of
> type *regexp.Regexp) is not constant
>
That error indicates that you wrote `const ExtRegex = ....`
>
> while
> const pat = "((M|m)(p|P)(3|4))|((F|f)(L|l)(A|a)(C|c))$"
> var ExtRegex = regexp.MustCompile(pat)
>
> Works fine.
> So, why can't the regexp be a constant?
>
Only primitive types are allowed to be constant in go
<https://go.dev/ref/spec#Constants>
> Is there some state that is kept in the regexp.Regexp store?
>
It used to, but that was removed in an earlier version (see the deprecation
notice on regexp.Copy <https://pkg.go.dev/regexp#Regexp.Copy>)
>
> And perhaps more importantly, what is the proper go style to
> have a compiled regexp?
> I could put the var statement outside all blocks, so its in effect
> a package variable. But I think having package variable is bad form.
>
For cases like this, I tend to create an unexported package variable that's
initialized with MustCompile.
package-level variables are generally bad-form, but there are cases where
you have mostly-readonly things that must be initialized at startup in
which they make sense. (constant-initialized regexps are one of these)
>
> I'm using the regexp in a loop for all the strings in all the files in a
> directory tree.
> I really don't want to compile them for ever pass thru the lines
>
> Thanks
> Pat
>
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