Solution from https://www.facebook.com/groups/584867114995854/

benchcmp-1

$ git diff
diff --git a/check.go b/check.go
index 6a25548..485f92e 100644
--- a/check.go
+++ b/check.go
@@ -2,6 +2,10 @@ package main

 func check(input string) bool {
        for i := 0; i < len(input); i++ {
+               if input[len(input)-1] == '{' {
+                       return false
+               }
+
                b := input[i]

                if b == 'x' {
@@ -9,9 +13,6 @@ func check(input string) bool {
                }

                if b == '{' {
-                       if i == len(input)-1 {
-                               return false
-                       }
                        if input[i+1] == 'x' {
                                return false
                        }
$ benchcmp before.out after.out
benchmark                                          old ns/op     new ns/op     delta
BenchmarkCheck/xxxxxxxxxxx{}xxxxxxxxxxxxxxx-8      21.7          29.5          +35.94%
BenchmarkCheck/xxxxxxxxxxx{}xxxxxxx{}xxxxx-8       21.2          27.9          +31.60%
BenchmarkCheck/xxxxxxxxxxxxx-8                     12.3          15.7          +27.64%
BenchmarkCheck/xxxxxxxxxxx{xxxxxxx{}xxxxx-8        10.9          14.0          +28.44%
BenchmarkCheck/xxxxxxxxxxx{}xxxxxxx{x}xxxxx-8      17.1          22.8          +33.33%
BenchmarkCheck/xxxxxxxxxxx{x}xxxxxxxxxxxxxxx-8     11.3          14.1          +24.78%
BenchmarkCheck/xxxxxxxxxxx}{xxxxxxxxxxxxxxx-8      11.5          14.3          +24.35%
BenchmarkCheck/{xxxxxxxxxxxxxxx-8                  3.87          3.58          -7.49%
BenchmarkCheck/xxxxxxxxxxxxxxx}x-8                 13.8          18.1          +31.16%
BenchmarkCheck/xxxxxxxxxx{-8                       10.3          3.14          -69.51%
BenchmarkCheck/}xxxxxxxxxx-8                       3.96          3.72          -6.06%
BenchmarkCheck/x{}xxx{xx-8                         8.96          10.0          +11.61%
$

check.go

package main

func check(input string) bool {
	if input[len(input)-1] == '{' || input[0] == '}' {
		return false
	}

	for i := 0; i < len(input); i++ {
		b := input[i]

		if b == 'x' {
			continue
		}

		if b == '{' {
			if input[i+1] == 'x' || input[i+1] == '{' {
				return false
			}
			i++
			continue
		}

		if b == '}' {
			if input[i-1] == '{' {
				continue
			}
			return false
		}
	}
	return true
}

check_test.go

package main

import "testing"

var tcs = []struct {
	input  string
	output bool
}{
	{"xxxxxxxxxxx{}xxxxxxxxxxxxxxx", true},
	{"xxxxxxxxxxx{}xxxxxxx{}xxxxx", true},
	{"xxxxxxxxxxxxx", true},
	{"xxxxxxxxxxx{xxxxxxx{}xxxxx", false},
	{"xxxxxxxxxxx{}xxxxxxx{x}xxxxx", false},
	{"xxxxxxxxxxx{x}xxxxxxxxxxxxxxx", false},
	{"xxxxxxxxxxx}{xxxxxxxxxxxxxxx", false},
	{"{xxxxxxxxxxxxxxx", false},
	{"xxxxxxxxxxxxxxx}x", false},
	{"xxxxxxxxxx{", false},
	{"}xxxxxxxxxx", false},
	{"x{}xxx{xx", false},
}

func TestCheck(t *testing.T) {
	for _, tc := range tcs {
		t.Run(tc.input, func(t *testing.T) {
			if ok := check(tc.input); ok != tc.output {
				t.Errorf("expected %v got, %v", tc.output, ok)
			}
		})
	}
}

func BenchmarkCheck(b *testing.B) {
	for _, tc := range tcs {
		b.Run(tc.input, func(b *testing.B) {
			for i := 0; i < b.N; i++ {
				check(tc.input)
			}
		})
	}
}