The ValidPattern attribute accepts a string value that matches a valid .NET Regular Expression for that datatype.   Regular expressions are commonly used to enforce rules regarding the the types of characters and their placement in a field.

For example it is common to use a regular expression to force an email field to include the "@" character and a valid top level domain such as '.com' or '.net'.

Please refer to the Microsoft Developer Network for full documentation regarding the formation of Regular Expressions.

The following example forces the Friendly field to only use valid lowercase ascii characters with no spaces:
<Column Id="Friendly" Type="VarChar" Length="255" ValidPattern="^([a-z0-9]+(-[a-z0-9]+)*)?$" FriendlySource="Title"/>