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How to validate if input in input field has lowercase letters only using express-validator ?

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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 lowercase letters are allowed. We can also validate these input fields to accept only lowercase letters 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 validation isLowercase() 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 uppercase letters.

Filename – index.js

const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateTagName } = require('./validator')
const formTemplet = require('./form')
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
app.get('/', (req, res) => {
// Post route to handle form submission logic and
  async (req, res) => {
    const errors = validationResult(req)
      return res.send(formTemplet({errors}))
    const {email, name, tname, addr, phone} = req.body
    // New record
    await repo.create({
      'Tag name':tname,
'<strong>Information saved successfully!</strong>')
// 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
  // Get all existing records
  async getAll(){
    return JSON.parse(
      await fs.promises.readFile(this.filename, {
        encoding : 'utf8'
  // Create new record
  async create(attrs){
    // Fetch all existing records
    const records = await this.getAll()
    // All the existing records with new 
    // record push back to database
    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 – form.js: This file contains logic to show the form to submit the data.

const getError = (errors, prop) => {
  try {
    return errors.mapped()[prop].msg
  } catch (error) {
    return ''
module.exports = ({errors}) => {
  return `
<!DOCTYPE html>
  <link rel='stylesheet' href=
    div.columns {
      margin-top: 100px;
    .button {
      margin-top: 10px
  <div class='container'>
    <div class='columns is-centered'>
      <div class='column is-5'>
        <form action='/info' method='POST'>
              <label class='label' id='email'>
            <input class='input' type='text' name='email'
              placeholder='' for='email'>
              <label class='label' id='name'>Name</label>
            <input class='input' type='text' name='name' 
              placeholder='Vinit singh' for='name'>
              <label class='label' id='tname'>
                Tag Name
            <input class='input' type='text' name='tname' 
              placeholder='solohunting' for='tname'>
            <p class="help is-danger">
              ${getError(errors, 'tname')}
              <label class='label' id='addr'>
            <input class='input' type='text' name='addr' 
              placeholder='Howrah, West Bengal' for='addr'>
              <label class='label' id='phone'>
                Phone Number
            <input class='input' type='text' name='phone' 
              placeholder='1111122222' for='phone'>
            <button class='button is-primary'>


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

const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateTagName : check('tname')
    // To delete leading and triling space
    // Validate name to accept only
    // lowercase letters
    // Custom message
    .withMessage('Must be all small letters')   


Filename – package.json

package.json file




Attempt to submit form data when tag name input field has not all lowercase letters

Response when attempt to submit form data where tag name input field has not all lowercase letters

Attempt to submit form data when tag name input field has all lowercase letters

Response when attempt to submit form data where tag name input field has all lowercase letters

Database after successful form submission:

Database after successful submission of form

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

Last Updated : 13 Aug, 2020
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