regex_tester.go

package main

import (
	"fmt"
	"math/rand"
	"regexp"
	"time"
)

func getRandomBytes(n uint32) []byte {
	rand.Seed(0)
	toReturn := make([]byte, n)
	for i := 0; i < len(toReturn); i++ {
		toReturn[i] = byte(rand.Int())
	}
	return toReturn
}

var tests = []*regexp.Regexp{
	regexp.MustCompile(`abcdef ghijklmnopqrstuv`),
	regexp.MustCompile(`abcdef.{0,20}ghijklmnopqrstuv`),
	regexp.MustCompile(`(?i)abcdef.{0,20}ghijklmnopqrstuv`),
	regexp.MustCompile(`(abc|def).{0,20}ghijklmnopqrstuv`),
}

func main() {
	fmt.Printf("Generating random data... ")
	data := getRandomBytes(15 * 1024 * 1024)
	fmt.Printf("done.\n")
	matched := false
	var t time.Time
	var elapsed time.Duration
	for i, re := range tests {
		t = time.Now()
		matched = re.Match(data)
		elapsed = time.Now().Sub(t)
		fmt.Printf("Test %d took %f seconds.\n", i, elapsed.Seconds())
		if matched {
			fmt.Printf("How did a match occur in random data?\n")
		}
	}
}

Output for me:

 Generating random data... done.
 Test 0 took 0.003473 seconds.
 Test 1 took 0.003431 seconds.
 Test 2 took 1.799499 seconds.
 Test 3 took 2.017934 seconds.

Go Version:
go version go1.4 linux/amd64

2017 update: case-insensitive regexes or regexes with | are still much slower:

Generating random data... done.
Test 0 took 0.005006 seconds.
Test 1 took 0.005000 seconds.
Test 2 took 1.326073 seconds.
Test 3 took 1.950103 seconds.

Go Version:
go version go1.8 windows/amd64