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Mastering Regular Expressions in PHP
Author : Dennis Pallett
What are
Regular Expressions? A regular expression is a pattern
that can match various text strings. Using regular expressions
you can find (and replace) certain text patterns, for example
"all the words that begin with the letter A" or "find only
telephone numbers". Regular expressions are often used in
validation classes, because they are a really powerful tool to
verify e-mail addresses, telephone numbers, street addresses,
zip codes, and more.
In this tutorial I will show you how regular expressions work in
PHP, and give you a short introduction on writing your own
regular expressions. I will also give you several example
regular expressions that are often used. Regular Expressions in
PHP Using regex (regular expressions) is really easy in PHP, and
there are several functions that exist to do regex finding and
replacing. Let's start with a simple regex find.
Have a look at the documentation of the preg_match function. As you
can see from the documentation, preg_match is used to perform a
regular expression. In this case no replacing is done, only a
simple find. Copy the code below to give it a try. <?php
// Example string $str = "Let's find the stuff
<bla>in between</bla> these two previous
brackets";
// Let's perform the regex $do =
preg_match("/<bla>(.*)</bla>/", $str,
$matches);
// Check if regex was successful if ($do = true) { // Matched
something, show the matched string echo
htmlentities($matches['0']);
// Also how the text in between the tags echo '<br />' .
$matches['1']; } else { // No Match echo "Couldn't find a
match"; }
?> After having run the code, it's probably a good idea
if I do a quick run through the code. Basically, the whole core
of the above code is the line that contains the preg_match. The
first argument is your regex pattern. This is probably the most
important. Later on in this tutorial, I will explain some basic
regular expressions, but if you really want to learn regular
expression then it's best if you look on Google for specific
regular expression examples.
The second argument is the subject string. I assume that needs
no explaining. Finally, the third argument can be optional, but
if you want to get the matched text, or the text in between
something, it's a good idea to use it (just like I used it in
the example). The preg_match function stops after it has found
the first match. If you want to find ALL matches in a string,
you need to use the preg_match_all
function. That works pretty much the same, so there is no
need to separately explain it.
Now that we've had finding, let's do a find-and-replace, with
the preg_replace
function. The preg_replace function works pretty similar to
the preg_match function, but instead there is another argument
for the replacement string. Copy the code below, and run it.
<?php
// Example string $str = "Let's replace the
<bla>stuff between</bla> the bla brackets";
// Do the preg replace $result = preg_replace
("/<bla>(.*)</bla>/", "<bla>new
stuff</bla>", $str);
echo htmlentities($result); ?> The result would then be
the same string, except it would now say 'new stuff' between the
bla tags. This is of course just a simple example, and more
advanced replacements can be done.
You can also use keys in the replacement string. Say you still
want the text between the brackets, and just add something? You
use the $1, $2, etc keys for those. For example: <?php
// Example string $str = "Let's replace the
<bla>stuff between</bla> the bla brackets";
// Do the preg replace $result = preg_replace
("/<bla>(.*)</bla>/", "<bla>new
stuff (the old: $1)</bla>", $str);
echo htmlentities($result); ?> This would then print
"Let's replace the new stuff (the old: stuff between)
the bla brackets". $2 is for the second "catch-all", $3 for the
third, etc.
That's about it for regular expressions. It seems very
difficult, but once you grasp it is extremely easy yet one of
the most powerful tools when programming in PHP. I can't count
the number of times regex has saved me from hours of coding
difficult text functions.
An
Example What would a good tutorial be without some real
examples? Let's first have a look at a simple e-mail validation
function. An e-mail address must start with letters or numbers,
then have a @, then a domain, ending with an extension. The
regex for that would be something like this:
^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$
Let me quickly explain that regex. Basically, the first part
says that it must all be letters or numbers. Then we get the @,
and after that there should be letters and/or numbers again (the
domain). Finally we check for a period, and then for an
extension. The code to use this regex looks like this:
<?php
// Good e-mail $good = "john@example.com";
// Bad e-mail $bad = "blabla@blabla";
// Let's check the good e-mail if
(preg_match("/^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$
/", $good)) { echo "Valid e-mail"; } else { echo
"Invalid e-mail"; }
echo '<br />';
// And check the bad e-mail if
(preg_match("/^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$
/", $bad)) { echo "Valid e-mail"; } else { echo
"Invalid e-mail"; }
?> The result of this would be "Valid E-mail. Invalid
E-mail", of course. We have just checked if an e-mail address is
valid. If you wrap the above code in a function, you've got
yourself a e-mail validation function. Keep in mind though that
the regex isn't perfect: after all, it doesn't check whether the
extension is too long, does it? Because I want to keep this
tutorial short, I won't give the full fledged regex, but you can
find it easily via Google.
Another
Example Another great example would be a telephone
number. Say you want to verify telephone numbers and make sure
they were in the correct format. Let's assume you want the
numbers to be in the format of xxx-xxxxxxx. The code would look
something like this: <?php
// Good number $good = "123-4567890";
// Bad number $bad = "45-3423423";
// Let's check the good number if
(preg_match("/d{3}-d{7}/", $good)) { echo "Valid
number"; } else { echo "Invalid number"; }
echo '<br />';
// And check the bad number if
(preg_match("/d{3}-d{7}/", $bad)) { echo "Valid
number"; } else { echo "Invalid number"; }
?> The regex is fairly simple, because we use d. This
basically means "match any digit" with the length behind it. In
this example it first looks for 3 digits, then a '-' (hyphen)
and finally 7 digits. Works perfectly, and does exactly what we
want.
What exactly is
possible with Regular Expressions? Regular expressions
are actually one of the most powerful tools in PHP, or any other
language for that matter (you can use it in your mod_rewrite
rules as well!). There is so much you can do with regex, and
we've only scratched the surface in this tutorial with some very
basic examples.
If you really want to dig into regex I suggest you search on
Google for more tutorials, and try to learn the regex syntax. It
isn't easy, and there's quite a steep learning curve (in my
opinion), but the best way to learn is to go through a lot of
examples, and try to translate them in plain English. It really
helps you learn the syntax.
In the future I will dedicate a complete article to strictly
examples, including more advanced ones, without any explanation.
But for now, I can only give you links to other tutorials: The
30 Minute Regex Tutorial Regular-Expressions.i
nfo
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