How to check if there is a special character in string or if a character is a special character in GoLang

After reading a string from the input, I need to check if there is a special character in it

You can use strings.ContainsAny to see if a rune exists:

package main

import (
  "fmt"
  "strings"
)

func main() {
  fmt.Println(strings.ContainsAny("Hello World", ",|"))
  fmt.Println(strings.ContainsAny("Hello, World", ",|"))
  fmt.Println(strings.ContainsAny("Hello|World", ",|"))
}

Or if you want to check if there are only ASCII characters, you can use strings.IndexFunc:

package main

import (
	"fmt"
	"strings"
)

func main() {
	f := func(r rune) bool {
		return r < 'A' || r > 'z'
	}
	if strings.IndexFunc("HelloWorld", f) != -1 {
		fmt.Println("Found special char")
	}
	if strings.IndexFunc("Hello World", f) != -1 {
		fmt.Println("Found special char")
	}
}

Depending on your definition of special character, the simplest solution would probably to do a for range loop on your string (which yield runes instead of bytes), and for each rune check if it is in your list of allowed/forbidden runes.

See Strings, bytes, runes and characters in Go for more about the relations between string, bytes and runes.

Playground example

package main

var allowed = []rune{'a','b','c','d','e','f','g'}

func haveSpecial(input string) bool {
	for _, char := range input {
		found := false
		for _, c := range allowed {
			if c == char {
				found = true
				break
			}
		}
		if !found {
			return true
		}
	}
	return false
}

func main() {
	cases := []string{
		"abcdef",
		"abc$€f",
	}
	for _, input := range cases {
		if haveSpecial(input) {
			println(input + ": NOK")
		} else {
			println(input + ": OK")
		}
	}
}

You want to use the unicode package, which has a nice function to check for symbols.

https://golang.org/pkg/unicode/#IsSymbol

package main

import (
	"fmt"
	"unicode"
)

func hasSymbol(str string) bool {
	for _, letter := range str {
		if unicode.IsSymbol(letter) {
			return true
		}
	}
	return false
}

func main() {	
	var strs = []string {
		"A quick brown fox",
		"A+quick_brown<fox",
	}
	
	for _, str := range strs {
		if hasSymbol(str) { 
			fmt.Printf("String '%v' contains symbols.\n", str)
		} else {
			fmt.Printf("String '%v' did not contain symbols.\n", str)	
		}
	}
}

This will provide the following output:

String 'A quick brown fox' did not contain symbols.
String 'A+quick_brown<fox' contains symbols.

I ended up doing something like this

alphabet := "abcdefghijklmnopqrstuvwxyz"
alphabetSplit := strings.Split(alphabet, "")
inputLetters := strings.Split(input, "")

for index, value := range inputLetters {
        special:=1
        for _, char :=range alphabetSplit{
            if char == value {
                special = 0
                break
            }
        }

It might have anything wrong because since I used it to something specific i had to edit to post it here