Regular expressions can be used to identify entities in a document. We will investigate two general approaches:

• The first one uses regular expressions as supported by Java. These can be useful in situations where the entities are relatively simple and consistent in their form.
• The second approach uses classes designed to specifically use regular expressions. To demonstrate this, we will use LingPipe's RegExChunker class.

When working with regular expressions, it is advantageous to avoid reinventing the wheel. There are many sources for predefined and tested expressions. One such library can be found at http://regexlib.com/Default.aspx. We will use several of the regular expressions in this library for our examples.

To test how well these approaches work, we will use the following text for most of our examples:

private static String regularExpressionText 
    = "He left his email address ([email protected]) and his " 
    + "phone number,800-555-1234. We believe his current address " 
    + "is 100 Washington Place, Seattle, CO 12345-1234. I " 
    + "understand you can also call at 123-555-1234 between " 
    + "8:00 AM and 4:30 most days. His URL is http://example.com " 
    + "and he was born on February 25, 1954 or 2/25/1954.";