WebReference.com - PHP and Regular Expressions 101 (4/5)

PHP and Regular Expressions 101

The space character

To match the space character in a search string, we use the predefined Posix class,
[[:space:]]. The square brackets indicate a related set of sequential characters,
and ":space:" is the actual class to match (which, in this case, is any white
space character). White spaces include the tab character, the new line character, and the space
character. Alternatively, you could use one space character (" ") if the search string must
contain just one space and not a tab or new line character. In most circumstances I prefer to
use ":space:" because it signifies my intentions a bit better than a single space
character, which can easy be overlooked. There are several Posix-standard predefined classes that
we can match as part of a regular expression, including [:alnum:], [:digit:],
[:lower:], etc. A complete list is available
here.

We can match a single space character like this:

<?php echo ereg("Mitchell[[:space:]]Harper", "Mitchell Harper"); ?>

We could also tell the regular expression engine to match either no spaces or one space by using the
? character after the expression, like this:

<?php echo ereg("Mitchell[[:space:]]?Harper", "MitchellHarper"); ?>

Grouping patterns

Related patterns can be grouped together between square brackets. It's really easy to specify
that a lower case only or upper case only sequence of characters should exist as part of the search
string using [a-z] and [A-Z], like this:

We can also tell the regular expression engine that we expect either lower case or upper case
characters. We do this by joining the [a-z] and [A-Z] patterns:

<?php echo ereg("^[a-zA-Z]+$", "JohnDoe"); ?>

In the example above, it would make sense if we could match "John Doe," and not "JohnDoe." We can
use the following regular expression to do so:

^[a-zA-Z]+[[:space:]]{1}[a-zA-Z]+$

It's just as easy to search for a numerical string of characters:

<?php echo ereg("^[0-9]+$", "12345"); ?>

Grouping terms

It's not only search patterns that can be grouped together. We can also group related
search terms together using parentheses:

<?php echo ereg("^(John|Jane).+$", "John Doe"); ?>

In the example above, we have a beginning of string character, followed by "John" or "Jane", at
least one other character, and then the end of string character. So ...

<?php echo ereg("^(John|Jane).+$", "Jane Doe"); ?>

... would also match our search pattern.

Special character circumstances

Because several characters are used to actually specify the grouping or syntax of a search
pattern, such as the parentheses in (John|Jane), we need a way to tell the regular
expression engine to ignore these characters and to process them as if they were part of the
string being searched and not part of the search expression. The method we use to do this
is called "character escaping" and involves propending any "special symbols" with a
backslash. So, for example, if I wanted to include the or symbol '|' in my search, then
I could do so like this:

<?php echo ereg("^[a-zA-z]+\|[a-zA-z]+$", "John|Jane"); ?>

There are only a handful of symbols that you have to escape. You must
escape ^, $, (, ), ., [,
|, *, ?, +, \ and {.

Hopefully you've now gotten a bit of a feel for just how powerful regular expressions actually are.
Let's now take a look at two examples of using regular expressions to validate a string of data.