MIME Type

const re = new RegExp("^([a-z]+)\\/([a-z0-9][a-z0-9!#$&\\-^_.]*)(?:;\\s*[a-z]+=\\S+)*$", "i");
const input = "application/json";
const match = input.match(re);
console.log(match ? match[0] : null);

import re

pattern = re.compile(r"^([a-z]+)\/([a-z0-9][a-z0-9!#$&\-^_.]*)(?:;\s*[a-z]+=\S+)*$", re.IGNORECASE)
input_text = "application/json"
for m in pattern.finditer(input_text):
    print(m.group(0))

package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`(?i)^([a-z]+)\/([a-z0-9][a-z0-9!#$&\-^_.]*)(?:;\s*[a-z]+=\S+)*$`)
	input := `application/json`
	for _, match := range re.FindAllString(input, -1) {
		fmt.Println(match)
	}
}