MySQL Cookbook

Validation by Pattern Matching

Problem

You need to compare a value to a set of values that is difficult to specify literally without writing a really ugly expression.

Solution

Use pattern matching.

Discussion

Pattern matching is a powerful tool for validation because it allows you to test entire classes of values with a single expression. You can also use pattern tests to break up matched values into subparts for further individual testing, or in substitution operations to rewrite matched values. For example, you might break up a matched date into pieces so that you can verify that the month is in the range from 1 to 12 and the day is within the number of days in the month. Or you might use a substitution to reorder MM-DD-YY or DD-MM-YY values into YY-MM-DD format.

The next few sections describe how to use patterns to test for several types of values, but first let’s take a quick tour of some general pattern-matching principles. The following discussion focuses on Perl’s regular expression capabilities. Pattern matching in PHP and Python is similar, though you should consult the relevant documentation for any differences. For Java, the ORO pattern matching class library offers Perl-style pattern matching; Appendix A indicates where you can get it.

In Perl, the pattern constructor is /pat/:

$it_matched = ($val =~ /pat/);    # pattern match

Put an i after the /pat/ constructor to make the pattern match case insensitive:

$it_matched = ($val =~ /pat/i);   # case-insensitive match

To use a character ...