Using Regular Expressions with MySQL

The following metacharacters are used with the LIKE keyword in an SQL WHERE clause.

Metacharacter	Comment
_	Matches any single character
%	Matches zero or more characters

The following metacharacters are used with the REGEXP keyword in a WHERE clause.

Metacharacter	Comment
^	The beginning-of-field (column) position metacharacter.
$	The end-of-field (column) position metacharacter.
[ ...]	Character class. Supported, including ranges.
[^ ... ]	Negated character class.
?	Quantifier. The preceding character or group is
	optional.
*	Quantifier. The preceding character or group occurs
	zero or more times.
+	Quantifier. The preceding character or group occurs
	one or more times.
{n,m}	Quantifier. The preceding character or group occurs at
	least n times and no more than m times.
|	Supports alternation. The | metacharacter separates
	mutually exclusive options.

Lookahead, lookbehind, and back references are not supported in MySQL 4.0.

Using the _ and % Metacharacters

The _ and % metacharacters are SQL metacharacters. They are used in a WHERE clause with the LIKE keyword. The _ metacharacter matches a single character and is similar in meaning to the period metacharacter in standard regular expression syntax. The % metacharacter matches zero or more characters and is equivalent to .* in standard regular expression syntax.

In MySQL, matching using the _ and % metacharacters is case insensitive.

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Try It Out

The _ and % Metacharacters

The following instructions assume that you have a command window open, with an operating system command prompt, and that the current directory is the MySQL bin directory.

1.Start the mysql utility, and at the mysql command line, issue the following command to switch to the BRegExp database:

USE BRegExp;

First, you will use the _ metacharacter to select rows where the value of the last name begins with B, has any single character, and then has the character sequence rns.

2.Type the following command at the mysql command prompt:

SELECT LastName, FirstName

FROM People

WHERE LastName LIKE ‘B_rns’

;

Figure 17-4 shows the results after this step.

Figure 17-4

3.When the _ metacharacter is used, there are implicitly beginning-of-field and end-of-field metacharacters, too. You can see that if you modify the code to remove the final s in the pattern. At the mysql command prompt, type the following:

SELECT LastName, FirstName FROM People

WHERE LastName LIKE ‘B_rn’

;

As shown in Figure 17-5, the result is now an empty set. In other words, there are no matches. The MySQL regular expression engine treats the pattern B_rn as though it were ^B_rn$.

Figure 17-5

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Using Regular Expressions with MySQL

4.The % metacharacter can allow you to see only rows where the person’s last name begins with the character sequence Cl. At the mysql command prompt, type the following command:

SELECT LastName, FirstName FROM People

WHERE LastName LIKE ‘Cl%’

;

As you can see in Figure 17-6, only rows where the last name begins with the character sequence Cl are returned.

Figure 17-6

5.You can also use the % metacharacter to match patterns that occur in DATE type columns. You can use this, for example, to match rows where the date of birth is in the 1950s. The pattern 195% allows you to match such dates. On this occasion, use the ORDER BY clause to put the returned rows into date order.

Type the following code at the mysql command prompt:

SELECT FirstName, LastName, DateOfBirth FROM People

WHERE DateOfBirth LIKE ‘195%’ ORDER BY DateOfBirth

;

6.Inspect the results, as shown in Figure 17-7. Only people whose date of birth begins with the character sequence 195 are displayed.

Figure 17-7

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Testing Matching of Literals: _ and % Metacharacters

As well as using the LIKE keyword with the _ and % metacharacters to match data already held in a database, you can directly explore whether a character sequence and a pattern matches. This allows you to test whether a desired string is matched by the pattern you are constructing.

The syntax is as follows:

SELECT “TheString” LIKE “The Pattern”;

The delimiters of the string and of the pattern can be paired double quotes, as shown in the preceding code, or can be paired apostrophes, as in the following:

SELECT ‘TheString’ LIKE ‘The Pattern’;

Try It Out

Selecting Matching of Literals

Check whether the pattern Fr% matches the character sequence Fred.

1.Type the following at the mysql command line:

SELECT “Fred” LIKE “Fr%”;

2.Type the following at the mysql command line:

SELECT “Bid” LIKE “B_d”

3.Inspect the results. Figure 17-8 shows the result after Steps 1 and 2. The figure 1 in the result indicates that there is a match, representing a Boolean value of True.

Figure 17-8

When matching is unsuccessful, a value of 0 is displayed in the results.

4.Type the following code at the mysql command line:

SELECT “Fred” LIKE “B_d”;

5.Inspect the results. You can be confident that the character sequence Fred is not matched by the pattern B_d, as when a value of 0 is returned, matching has failed. If you are testing a regular expression that you expect to match, you need to do further work to get it right.

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Using Regular Expressions with MySQL

Using the REGEXP Keyword and

Metacharacters

MySQL provides an additional keyword that you can use to apply regular expression functionality — the REGEXP keyword. Like the LIKE keyword, the REGEXP keyword is used in a SELECT statement’s WHERE clause.

The RLIKE keyword is a synonym for REGEXP.

For the examples in this section, you need to extend the People table created earlier and create an Employees table, which includes SSN, Department, Skills, and Comments columns. The script to create and populate the Employees table is shown here:

USE BRegExp;

CREATE TABLE Employees

(ID INT PRIMARY KEY AUTO_INCREMENT, LastName VARCHAR(20),

FirstName VARCHAR(20), DateOfBirth DATE,

SSN VARCHAR(11), Department VARCHAR(18), Skills VARCHAR(50), Comments VARCHAR(100));

INSERT INTO Employees

(ID, LastName, FirstName, DateOfBirth, SSN, Department, Skills, Comments) VALUES

(NULL, ‘Smith’, ‘George’, ‘1959-11-11’, ‘123-45-6789’, ‘Data Management’, ‘Analysis Services, Business Intelligence, Data Transformation Services’, ‘Good skills in SQL Server 2000. Can be grumpy at times.’),

(NULL, ‘Armada’, ‘Francis’, ‘1971-03-08’, ‘881-32-8913’, ‘Sales’, NULL, ‘Effective salesman. Particularly good at relating to the business needs of clients.’),

(NULL, ‘Schmidt’, ‘Georg’, ‘1981-10-09’, ‘456-12-1234’, ‘Admin’, NULL, ‘Effective head of Admin Department. Good communicator.’),

(NULL, ‘Clingon’, ‘David’, ‘1944-11-01’, ‘234-59-3489’, ‘Data Management’, ‘DBA, SQL DMO’, ‘Good database administrator. Lots of experience.’),

(NULL, ‘Dalek’, ‘Eve’, ‘1953-04-04’, ‘345-19-8822’, ‘Sales’, NULL, ‘Good sales record. Technically informed.’),

(NULL, ‘Bush’, ‘Harold’, ‘1939-11-08’, ‘378-12-0021’, ‘Public Relations’, NULL, ‘An old hand. Handled virus crisis excellently last year.’),

(NULL, ‘Burns’, ‘Geoffrey’, ‘1960-08-02’, ‘000-12-3872’, ‘Development’, ‘C#,

.NET’, ‘Good .NET programmer. Can lack vision of bigger picture at times.’), (NULL, ‘Builth’, ‘Wellstone’, ‘1947-10-05’, ‘009-348-234’, ‘Development’,

‘VB.NET, .NET, ADO.NET’, ‘Sound. Useful member of team.’),

(NULL, ‘Thomas’, ‘Dylan’, ‘1984-07-07’, ‘310-23-3891’, ‘Data Management’, ‘DTS’, ‘Great guy for those data transformation jobs.’),

(NULL, ‘LLareggub’, ‘Dai’, ‘1950-11-02’, ‘210-23-4578’, ‘Data Processing’, ‘Data Transformation Services, SQL DMO’, ‘Good guy. Could be more proactive.’),

(NULL, ‘Barns’, ‘Samuel’, ‘1944-06-01’, ‘238-12-9999’, ‘International Sales’, ‘Good French and German skills.’, ‘Good salesman.’),

(NULL, ‘Claverhouse’, ‘Henry’, ‘1931-08-12’, ‘723-123-234’, ‘International Sales’, NULL, ‘Semi-retired now. Still effective though.’),

(NULL, ‘Litmus’, ‘Susie’, ‘1954-11-03’, ‘123-34-4888’, ‘Admin’, ‘Good organizer.’, ‘Deputy to Georg Schmidt.’);

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To run the Employees.sql script, at an operating system command line, type the following command:

mysql <C:\BRegExp\Ch17\Employees.sql

If the script has run successfully, the prompt should be displayed with no error messages showing.

For the quantifier examples, use the Parts table, which is created by the script Parts.sql, shown here:

USE BRegExp; CREATE TABLE Parts

(ID INT PRIMARY KEY AUTO_INCREMENT, PartNum VARCHAR(12),

Description VARCHAR(50)); INSERT INTO Parts

(ID, PartNum, Description) VALUES

(NULL, ‘ABC123’, ‘A basic widget.’), (NULL, ‘AAC123’, ‘A special widget.’), (NULL, ‘ABBC1234’, ‘A green widget.’), (NULL, ‘AAAAAAC2345’, ‘A purple thing.’), (NULL, ‘AAAAADD8899’, ‘A tartan widget’), (NULL, ‘BC123’, ‘A thin widget’),

(NULL, ‘ART987’, ‘An artistic widget’), (NULL, ‘XYZ345’, ‘A recent widget’), (NULL, ‘AB123’, ‘A super widget’), (NULL, ‘AC123’, ‘An exercise widget’), (NULL, ‘ABCD234567’, ‘A long widget’), (NULL, ‘STUV234’, ‘A late widget’), (NULL, ‘VWX7656’, ‘An automatic widget’),

(NULL, ‘NOP278’, ‘An opinionated widget’), (NULL, ‘A2345’, ‘An numeric widget’);

At the operating system command line, with the current directory being the MySQL bin directory, enter the following command:

mysql <C:\BRegExp\Ch17\Parts.sql

If the script runs without error messages, it is likely that it has run successfully. However, it makes sense to test that the Employees and Parts tables have been created and populated successfully.

Run the mysql utility. At the mysql prompt, type the following command to switch to the BRegExp database, which is where the Employees and Parts tables should have been created:

USE BRegExp;

Because of the number of columns in the Employees table and the length of the Skills and Comments columns, you will display the data in two parts. At the mysql command prompt, type the following:

SELECT ID, LastName, FirstName, DateOfBirth FROM Employees

;

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If the table has been created correctly, 13 rows should be displayed with data in all columns, including the automatically numbered ID column. Figure 17-9 shows the result you should see.

Figure 17-9

Run the following commands to test the other columns. First,

SELECT ID, SSN, Department, Skills FROM Employees

;

and then:

SELECT ID, Comments FROM Employees

;

Because of the length of the data in the Skills and Employees columns, some of the data will wrap from one line to another, resulting in an untidy appearance. You will have to scroll up a little to confirm that these columns have been correctly populated.

To confirm that the Parts table has been populated correctly, type the following command:

SELECT * FROM Parts

;

You don’t need to switch databases because, assuming that you have run the code to test the Employees table, you are already using the BRegExp database. Figure 17-10 shows the result if the Parts table has been correctly populated.

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