SharePoint Form Validation supports a client-side validation for SharePoint list field data. SharePoint Form Validation provides 4 standard validation types for input content check when users create a SharePoint list item in New-Form or edit an item in Edit Form. They are Required Validation, Regular Expression Validation, Compare Validation, and Range Validation.
Administrator can configure above four validations and customize warning error messages for most columns in a list. Required Field Validator is used for setting whether some column should not be null. Regular Expression Validator is used to check whether inputted content is against a regular expression. Range Validator makes effect when it is required that some value of data should be in a range. Compare Validator plays an important role when data are set to be compared with each other.
By default, SharePoint can do some simple validations to SharePoint list field data such as "Required Field" on server side when you submit form. But users often need list field data validations of various kinds on client side. For instance, when submitter creates a task and input data in a SharePoint task list form, administrator may want him to be noticed on the client side if the data of "Start Date" column is later than "End Date" column. In such a case, SharePoint Form Validation can help administrators achieve it.
SharePoint Form Validation can be used on almost all kinds of lists. For most columns in a list, users can apply standard validation methods such as "Required Validation", "Compare Validation", "Regular Expression Validation" and "Range Validation" on columns. SharePoint Form Validation can take effect in the SharePoint list item New Form page or Edit Form page.
Check whether the user has entered or selected value;
Check whether input value is against a regular expression;
Check whether input value is between two fixed values;
Compare input value with a fixed value or others
List frequently-used regular expressions in setting pages.