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How to validate if input in input field has alphanumeric characters only using express-validator ?

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  • Last Updated : 27 Dec, 2021

In HTML forms, we often required validation of different types. Validate existing email, validate password length, validate confirm password, validate to allow only integer inputs, these are some examples of validation. In a certain input field, only alphanumeric characters are allowed i.e. there not allowed any special characters. We can also validate these input fields to accept only alphanumeric characters using express-validator middleware.

Command to install express-validator:

npm install express-validator

Steps to use express-validator to implement the logic:

  • Install express-validator middleware.
  • Create a validator.js file to code all the validation logic.
  • Validate input by validateInputField: check(input field name) and chain on the validation isAlphanumeric() with ‘ . ‘
  • Use the validation name(validateInputField) in the routes as a middleware as an array of validations.
  • Destructure ‘validationResult’ function from express-validator to use it to find any errors.
  • If error occurs redirect to the same page passing the error information.
  • If error list is empty, give access to the user for the subsequent request.

Note: Here we use local or custom database to implement the logic, the same steps can be followed to implement the logic in a regular database like MongoDB or MySql.

Example: This example illustrates how to validate a input field to only allow the alphabets.

Filename – index.js

const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateUsername } = require('./validator')
const signupTemplet = require('./signup')
const app = express()
const port = process.env.PORT || 3000
// The body-parser middleware to parse form data
app.use(bodyParser.urlencoded({extended : true}))
// Get route to display HTML form to sign up
app.get('/signup', (req, res) => {
// Post route to handle form submission logic and
  async (req, res) => {
    const errors = validationResult(req)
    if(!errors.isEmpty()) {
      return res.send(signupTemplet({errors}))
    const {email, username, password} = req.body
    await repo.create({
    res.send('Sign Up successfully')
// Server setup
app.listen(port, () => {
  console.log(`Server start on port ${port}`)

Filename – repository.js: This file contains all the logic to create a local database and interact with it.

// Importing node.js file system module 
const fs = require('fs')
class Repository {
  constructor(filename) {
    // Filename where datas are going to store
    if(!filename) {
      throw new Error('Filename is required to create a datastore!')
    this.filename = filename
    try {
    } catch(err) {
      // If file not exist it is created with empty array
      fs.writeFileSync(this.filename, '[]')
  // Get all existing records
  async getAll(){
    return JSON.parse(
      await fs.promises.readFile(this.filename, {
        encoding : 'utf8'
  // Create new record
  async create(attrs){
    const records = await this.getAll()
    await fs.promises.writeFile(
      JSON.stringify(records, null, 2)   
    return attrs
// The 'datastore.json' file created at runtime 
// and all the information provided via signup form
// store in this file in JSON format.
module.exports = new Repository('datastore.json')

Filename – signup.js: This file contains logic to show sign up form.

const getError = (errors, prop) => {
  try {
    return errors.mapped()[prop].msg
  } catch (error) {
    return ''
module.exports = ({errors}) => {
  return `
    <!DOCTYPE html>
        <link rel='stylesheet' 
            margin-top: 100px;
            margin-top : 10px
        <div class='container'>
          <div class='columns is-centered'>
            <div class='column is-5'>
              <h1 class='title'>Sign Up<h1>
              <form method='POST'>             
                    <label class='label' id='email'>Email</label>
                  <input class='input' type='text' name='email' 
                  placeholder='Email' for='email'>
                    <label class='label' id='fn'>Username</label>
                  <input class='input' type='text' name='username'
                  placeholder='Username' for='username'>
                  <p class="help is-danger">${getError(errors, 'username')}</p>
                    <label class='label' id='password'>Password</label>
                  <input class='input' type='password' name='password' 
                  placeholder='Password' for='password'>
                  <button class='button is-primary'>Sign Up</button>

Filename – validator.js: This file contain all the validation logic(Logic to validate a input field to only allow the alphanumeric characters).

const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateUsername : check('username')
    // To delete leading and trailing space
    // Validate minimum length of password
    // Optional for this context
    // Custom message
    .withMessage('Username must be minimum 4 characters long')
    // Validate username to be alphanumeric
    // Custom message
    .withMessage('Username must be alphanumeric')

Filename – package.json

package.json file




Attempt to sign up when username input field is not alphanumeric

Response when attempt to sign up with username input field which is not alphanumeric

Attempt to sign up when username input field contains only alphanumeric character

Response when attempt to sign up with username input field which contains only alphanumeric characters

Database after successful Sign Up:

Database after successful Sign Up

Note: We have used some Bulma classes(CSS framework) in the signup.js file to design the content.

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