http://jsfiddle.net/gh/gist/library/pure/d58699789d445a2fd4f1/

fiddle.css

@import url('https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css');

fiddle.html

<div class="container">

  <form class="form-horizontal">
    <h1>Form</h1>

    <div class="form-group">
      <label class="col-sm-2 control-label" for="inputEmail">Email</label>
      <div class="col-sm-10" data-bind="validationElement: email">
        <input type="text" id="inputEmail" class="form-control" placeholder="Email" data-bind="textinput: email"/>
        <p class="text-danger" data-bind="validationMessage: email"></p>
      </div>
    </div>

    <div class="form-group well well-lg" >
      <div class="col-sm-offset-2 col-sm-10">
        <button type="submit" class="btn btn-primary">Submit</button>
        <button type="button" class="btn">Cancel</button>
      </div>
    </div>

  </form>

</div>

fiddle.js

$(function() {
    ko.validation.rules['emailLocalPartMaxLength'] = {
        validator: function (val, params) {
            if(ko.validation.utils.isEmptyVal(val)) {
                return true;
            }
            var normalizedVal = ko.validation.utils.isNumber(val) ? ('' + val) : val;
            var emails = normalizedVal.split('@')
            if(emails.length != 2) {
                return true;
            }
            return emails[0].length <= params;
        },
        message: 'message'
    };
    ko.validation.registerExtenders();

    function myViewModel() {
        var self = this;
        self.email = ko.observable().extend({
            email: {
                message: 'invalid format'
            },
            emailLocalPartMaxLength: {
                params: 10,
                message: 'local part too long'
            }
        });
    }

    ko.validation.init({
        insertMessages: false,
        decorateElement: true,
        errorElementClass: 'has-error'
    });
    var viewModel = ko.validatedObservable(new myViewModel());
    ko.applyBindings(viewModel);
})

fiddle.manifest

name: KnockoutJS Validation Sample 3
authors:
  - laughingman7743
resources:
  - http://knockoutjs.com/downloads/knockout-3.3.0.js
  - http://code.jquery.com/jquery-2.1.4.min.js
  - http://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js
  - http://cdnjs.cloudflare.com/ajax/libs/knockout-validation/2.0.3/knockout.validation.min.js
normalize_css: no
wrap: b
panel_js: 0
panel_css: 0