<div class="form-group">

   <label for="username" class="control-label">Username</label>

   <input type="text"

       class="form-control"

       id="username"

       ng-model="formModel.username"

       required="required"

       ng-pattern="/^[A-Za-z0-9_]{1,32}$/"

       ng-minlength="7"

       ng-pattern-err-type="badUsername"

    />

  </div>

可以用正则表达式