Source: lazyre.go in package internal/lazyregexp

Package lazyregexp is a thin wrapper over regexp, allowing the use of global regexp variables without forcing them to be compiled at init.

package lazyregexp

import (
	"os"
	"regexp"
	"strings"
	"sync"
)

Regexp is a wrapper around regexp.Regexp, where the underlying regexp will be compiled the first time it is needed.

type Regexp struct {
	str  string
	once sync.Once
	rx   *regexp.Regexp
}

func (r *Regexp) re() *regexp.Regexp {
	r.once.Do(r.build)
	return r.rx
}

func (r *Regexp) build() {
	r.rx = regexp.MustCompile(r.str)
	r.str = ""
}

func (r *Regexp) FindSubmatch(s []byte) [][]byte {
	return r.re().FindSubmatch(s)
}

func (r *Regexp) FindStringSubmatch(s string) []string {
	return r.re().FindStringSubmatch(s)
}

func (r *Regexp) FindStringSubmatchIndex(s string) []int {
	return r.re().FindStringSubmatchIndex(s)
}

func (r *Regexp) ReplaceAllString(src, repl string) string {
	return r.re().ReplaceAllString(src, repl)
}

func (r *Regexp) FindString(s string) string {
	return r.re().FindString(s)
}

func (r *Regexp) FindAllString(s string, n int) []string {
	return r.re().FindAllString(s, n)
}

func (r *Regexp) MatchString(s string) bool {
	return r.re().MatchString(s)
}

func (r *Regexp) SubexpNames() []string {
	return r.re().SubexpNames()
}

var inTest = len(os.Args) > 0 && strings.HasSuffix(strings.TrimSuffix(os.Args[0], ".exe"), ".test")

New creates a new lazy regexp, delaying the compiling work until it is first needed. If the code is being run as part of tests, the regexp compiling will happen immediately.

func New(str string) *Regexp {
	lr := &Regexp{str: str}

In tests, always compile the regexps early.

		lr.re()
	}
	return lr