Angular 2 custom validation for numbers and decimal values

Something like this should work

import { AbstractControl, ValidatorFn } from '@angular/forms';

export class NumberValidators {

  static isNumberCheck(): ValidatorFn {
    return  (c: AbstractControl): {[key: string]: boolean} | null => {
      let number = /^[.\d]+$/.test(c.value) ? +c.value : NaN;
      if (number !== number) {
        return { 'value': true };
      }

      return null;
    };
  }
}

can use ng-pattern validation

  ng-pattern="^[0-9]+(.[0-9]{0,2})?$"

can you please check once like this

  <label for="upperbeltposition" 
   class="col-md-6 col-form-label" 
   name="number">
     Upper Belt 
     Position (mm)
      <span class="required">*</span>
  </label>
   <div class="col-md-5">
   <input 
    class="form-control input-transparent " 
    id="upperbeltposition" 
    type="text" 
    formControlName="upperbeltposition"  
     data-parsley-trigger="blur"
    required="required" 
    data-parsley-type="number"/> 
    <span  
     ng-show="submitForm && platformxposition 
      .number.$error.pattern">This value should be a valid number
   </span>

  </div>

You must add your form name. and input field name.$error.pattern

Instead of creating a custom validator, you can also simply use the Validators.pattern

https://angular.io/api/forms/Validators#pattern:

['', [Validators.required, Validators.pattern(/^[.\d]+$/)]]