import { FormControl } from '@angular/forms';
import { Injectable } from '@angular/core'

export class ValidateBase{
 public static urlValidator(input: FormControl){
  let validateString = "(https";

  console.log(input.value);
  //set value 
  let v = input.value;

    if(v!=undefined&&v!="")
    {
     if(v.toLowerCase().match(validateString))
     {
      return null;
     }
     else{
     return {
      isUrl:true
     }
    }
    }
    return null;
 }
}

<form [formGroup]="detailForm" (ngSubmit)="submit()">
 <div>
  <label for="LinkedURL">LinkedURL:</label>
  <input type="LinkedURL" name="LinkedURL" id="LinkedURL" [formControl]="LinkedURL">
  <div class="col-xs-4 col-sm-4" [style.color]="(LinkedURL.touched&&LinkedURL.valid==false)" [hidden]="LinkedURL.untouched">
   <div [hidden]="!LinkedURL.hasError('maxlength')">LinkedURL can not be greater than 250 characters.</div>
   <div [hidden]="!LinkedURL.hasError('isUrl')">LinkedURL is not an url.</div>
   <div [hidden]="!LinkedURL.hasError('required')">Required field.</div>
   <!--Success!-->
   <div [style.color]="'green'" [hidden]="!LinkedURL.valid">Validate success!</div>
  </div>
 </div>
 <button type="submit" [disabled]="!detailForm.valid">Log In</button>
</form>

import { ValidateBase } from './../../Validators/Validator.base';
import { Component, OnInit } from '@angular/core';

import {
 FormGroup,
 FormBuilder,
 FormControl,
 Validators
 } from '@angular/forms';

@Component({
 selector: 'validate-component',
 templateUrl: 'validate.component.html'
 })

export class SweepstakesDetailComponent implements OnInit {

 private detailForm: FormGroup;
 private LinkedURL: FormControl;

 ngOnInit(): void {
  this.validateForm();
 }

 private validateForm(){

  this.LinkedURL = new FormControl('',[
   Validators.required,
   Validators.maxLength(250),
   ValidateBase.urlValidator
  ]);

  //form
  this.detailForm = this.formBuilder.group({
   LinkedURL:this.LinkedURL,
  });
 }
}