Using Regular Expressions with PHP

Using Regular Expressions with PHP

Regular expressions are a powerful tool for examining and modifying text. Regular expressions themselves, with a general pattern notation almost like a mini programming language, allow you to describe and parse text. They enable you to search for patterns within a string, extracting matches flexibly and precisely. However, you should note that because regular expressions are more powerful, they are also slower than the more basic string functions. You should only use regular expressions if you have a particular need.

This tutorial gives a brief overview of basic regular expression syntax and then considers the functions that PHP provides for working with regular expressions.

* The Basics
* Matching Patterns
* Replacing Patterns
* Array Processing

PHP supports two different types of regular expressions: POSIX-extended and Perl-Compatible Regular Expressions (PCRE). The PCRE functions are more powerful than the POSIX ones, and faster too, so we will concentrate on them.

The Basics

In a regular expression, most characters match only themselves. For instance, if you search for the regular expression "foo" in the string "John plays football," you get a match because "foo" occurs in that string. Some characters have special meanings in regular expressions. For instance, a dollar sign ($) is used to match strings that end with the given pattern. Similarly, a caret (^) character at the beginning of a regular expression indicates that it must match the beginning of the string. The characters that match themselves are called literals. The characters that have special meanings are called metacharacters.

The dot (.) metacharacter matches any single character except newline (\). So, the pattern h.t matches hat, hothit, hut, h7t, etc. The vertical pipe (|) metacharacter is used for alternatives in a regular expression. It behaves much like a logical OR operator and you should use it if you want to construct a pattern that matches more than one set of characters. For instance, the pattern Utah|Idaho|Nevada matches strings that contain "Utah" or "Idaho" or "Nevada". Parentheses give us a way to group sequences. For example, (Nant|b)ucket matches "Nantucket" or "bucket". Using parentheses to group together characters for alternation is called grouping.

If you want to match a literal metacharacter in a pattern, you have to escape it with a backslash.

To specify a set of acceptable characters in your pattern, you can either build a character class yourself or use a predefined one. A character class lets you represent a bunch of characters as a single item in a regular expression. You can build your own character class by enclosing the acceptable characters in square brackets. A character class matches any one of the characters in the class. For example a character class [abc] matches a, b or c. To define a range of characters, just put the first and last characters in, separated by hyphen. For example, to match all alphanumeric characters: [a-zA-Z0-9]. You can also create a negated character class, which matches any character that is not in the class. To create a negated character class, begin the character class with ^: [^0-9].

The metacharacters +, *, ?, and {} affect the number of times a pattern should be matched. + means "Match one or more of the preceding expression", * means "Match zero or more of the preceding expression", and ? means "Match zero or one of the preceding expression". Curly braces {} can be used differently. With a single integer, {n} means "match exactly n occurrences of the preceding expression", with one integer and a comma, {n,} means "match n or more occurrences of the preceding expression", and with two comma-separated integers {n,m} means "match the previous character if it occurs at least n times, but no more than m times".

Now, have a look at the examples:

Regular Expression Will match...
foo The string "foo"
^foo "foo" at the start of a string
foo$ "foo" at the end of a string
^foo$ "foo" when it is alone on a string
[abc] a, b, or c
[a-z] Any lowercase letter
[^A-Z] Any character that is not a uppercase letter
(gif|jpg) Matches either "gif" or "jpeg"
[a-z]+ One or more lowercase letters
[0-9\.\-] Аny number, dot, or minus sign
^[a-zA-Z0-9_]{1,}$ Any word of at least one letter, number or _
([wx])([yz]) wy, wz, xy, or xz
[^A-Za-z0-9] Any symbol (not a number or a letter)
([A-Z]{3}|[0-9]{4}) Matches three letters or four numbers

Perl-Compatible Regular Expressions emulate the Perl syntax for patterns, which means that each pattern must be enclosed in a pair of delimiters. Usually, the slash (/) character is used. For instance, /pattern/.

The PCRE functions can be divided in several classes: matching, replacing, splitting and filtering.

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Matching Patterns

The preg_match() function performs Perl-style pattern matching on a string. preg_match() takes two basic and three optional parameters. These parameters are, in order, a regular expression string, a source string, an array variable which stores matches, a flag argument and an offset parameter that can be used to specify the alternate place from which to start the search:
preg_match ( pattern, subject [, matches [, flags [, offset]]])

The preg_match() function returns 1 if a match is found and 0 otherwise. Let's search the string "Hello World!" for the letters "ll":
if (preg_match("/ell/", "Hello World!", $matches)) {
echo "Match was found
";
echo $matches[0];
}
?>

The letters "ll" exist in "Hello", so preg_match() returns 1 and the first element of the $matches variable is filled with the string that matched the pattern. The regular expression in the next example is looking for the letters "ell", but looking for them with following characters:
if (preg_match("/ll.*/", "The History of Halloween", $matches)) {
echo "Match was found
";
echo $matches[0];
}
?>

Now let's consider more complicated example. The most popular use of regular expressions is validation. The example below checks if the password is "strong", i.e. the password must be at least 8 characters and must contain at least one lower case letter, one upper case letter and one digit:
$password = "Fyfjk34sdfjfsjq7";

if (preg_match("/^.*(?=.{8,})(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).*$/", $password)) {
echo "Your passwords is strong.";
} else {
echo "Your password is weak.";
}
?>

The ^ and $ are looking for something at the start and the end of the string. The ".*" combination is used at both the start and the end. As mentioned above, the .(dot) metacharacter means any alphanumeric character, and * metacharacter means "zero or more". Between are groupings in parentheses. The "?=" combination means "the next text must be like this". This construct doesn't capture the text. In this example, instead of specifying the order that things should appear, it's saying that it must appear but we're not worried about the order.

The first grouping is (?=.*{8,}). This checks if there are at least 8 characters in the string. The next grouping (?=.*[0-9]) means "any alphanumeric character can happen zero or more times, then any digit can happen". So this checks if there is at least one number in the string. But since the string isn't captured, that one digit can appear anywhere in the string. The next groupings (?=.*[a-z]) and (?=.*[A-Z]) are looking for the lower case and upper case letter accordingly anywhere in the string.

Finally, we will consider regular expression that validates an email address:
$email = firstname.lastname@aaa.bbb.com;
$regexp = "/^[^0-9][A-z0-9_]+([.][A-z0-9_]+)*[@][A-z0-9_]+([.][A-z0-9_]+)*[.][A-z]{2,4}$/";

if (preg_match($regexp, $email)) {
echo "Email address is valid.";
} else {
echo "Email address is not valid.";
}
?>

This regular expression checks for the number at the beginning and also checks for multiple periods in the user name and domain name in the email address. Let's try to investigate this regular expression yourself.

For the speed reasons, the preg_match() function matches only the first pattern it finds in a string. This means it is very quick to check whether a pattern exists in a string. An alternative function, preg_match_all(), matches a pattern against a string as many times as the pattern allows, and returns the number of times it matched.

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Replacing Patterns

In the above examples, we have searched for patterns in a string, leaving the search string untouched. The preg_replace() function looks for substrings that match a pattern and then replaces them with new text. preg_replace() takes three basic parameters and an additional one. These parameters are, in order, a regular expression, the text with which to replace a found pattern, the string to modify, and the last optional argument which specifies how many matches will be replaced.
preg_replace( pattern, replacement, subject [, limit ])

The function returns the changed string if a match was found or an unchanged copy of the original string otherwise. In the following example we search for the copyright phrase and replace the year with the current.
echo preg_replace("/([Cc]opyright) 200(3|4|5|6)/", "$1 2007", "Copyright 2005");
?>

In the above example we use back references in the replacement string. Back references make it possible for you to use part of a matched pattern in the replacement string. To use this feature, you should use parentheses to wrap any elements of your regular expression that you might want to use. You can refer to the text matched by subpattern with a dollar sign ($) and the number of the subpattern. For instance, if you are using subpatterns, $0 is set to the whole match, then $1, $2, and so on are set to the individual matches for each subpattern.

In the following example we will change the date format from "yyyy-mm-dd" to "mm/dd/yyy":
echo preg_replace("/(\d+)-(\d+)-(\d+)/", "$2/$3/$1", "2007-01-25");
?>

We also can pass an array of strings as subject to make the substitution on all of them. To perform multiple substitutions on the same string or array of strings with one call to preg_replace(), we should pass arrays of patterns and replacements. Have a look at the example:
$search = array ( "/(\w{6}\s\(w{2})\s(\w+)/e",
"/(\d{4})-(\d{2})-(\d{2})\s(\d{2}:\d{2}:\d{2})/");

$replace = array ('"$1 ".strtoupper("$2")',
"$3/$2/$1 $4");

$string = "Posted by John | 2007-02-15 02:43:41";

echo preg_replace($search, $replace, $string);?>

In the above example we use the other interesting functionality - you can say to PHP that the match text should be executed as PHP code once the replacement has taken place. Since we have appended an "e" to the end of the regular expression, PHP will execute the replacement it makes. That is, it will take strtoupper(name) and replace it with the result of the strtoupper() function, which is NAME.

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Array Processing

PHP's preg_split() function enables you to break a string apart basing on something more complicated than a literal sequence of characters. When it's necessary to split a string with a dynamic expression rather than a fixed one, this function comes to the rescue. The basic idea is the same as preg_match_all() except that, instead of returning matched pieces of the subject string, it returns an array of pieces that didn't match the specified pattern. The following example uses a regular expression to split the string by any number of commas or space characters:
$keywords = preg_split("/[\s,]+/", "php, regular expressions");
print_r( $keywords );
?>

Another useful PHP function is the preg_grep() function which returns those elements of an array that match a given pattern. This function traverses the input array, testing all elements against the supplied pattern. If a match is found, the matching element is returned as part of the array containing all matches. The following example searches through an array and all the names starting with letters A-J:
$names = array('Andrew','John','Peter','Nastin','Bill');
$output = preg_grep('/^[a-m]/i', $names);
print_r( $output );
?>

How to Enable Image Support for PHP

How to Enable Image Support for PHP

Enabling support for images such as JPEG and PNG images on PHP should be relatively easy, unfortunately online documentation on how to do this is unclear. The following is a list of steps that you can take to enable PHP support for your site. You might want to do this if you're trying to enable thumbnail generation on MediaWiki and you encounter an error like Fatal error: Call to undefined function: imagecreatefromjpeg() .
For SLES 9 simply install the following rpms:

t1lib-1.3.1-569.1.i586.rpm
php4-gd-4.3.4-43.8.i586.rpm

These steps were performed on a Red Hat Enterprise Linux ES release 3 machine, running PHP 4.3.10 and Apache 2.0.52.


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[edit] Steps

1. Download and install the JPEG library from here. The file you want to download is jpegsrc.v6b.tar.gz. Unzip, extract, configure, make and install by executing these steps:

tar xvfz jpegsrc.v6b.tar.gz
cd jpeg-6b
./configure
make
make install

2. This will install the library in your /usr/local directory. Unfortunately if you plan on compiling GD (GD is the graphics library that PHP uses) from scratch you will also have to copy over the header files manually by doing this:

cp ./include/j*.h /usr/local/include

3. Only do this if you want to compile GD yourself.

4. Download and install the LIBPNG library. Even though you might not configure PHP using PNG support, you can't for some reason configure GD without it. Go here, scroll down to where you see "Source Code" and download a version, this example will use the .tar.gz version.

tar xvfz libpng-1.2.8-config.tar.gz
cd libpng-1.2.8-config
./configure
make
make install

5. Now that these 2 libraries are installed, it's time to compile PHP. If you already have PHP installed on your machine you may have to download the source all over again and recompile it. Go to PHP's home page and download the most recent versin of the 4.x source code. Extract it and configure. The following configure options enable MySQL and Apache libraries as well as the image libraries, if you have any other libraries that you wish to use, make sure to include them.

./configure --with-mysql --with-apxs2=/usr/sbin/apxs \
--with-gd --with-jpeg --with-jpeg-dir=/usr/local \
--with-png --with-png-dir=/usr/local --with-zlib-dir=/usr
(the backslahes denote a new line)
make
make install

6. After this, it's time to restart Apache:

httpd -k restart

7. If you get a warning about the PHP module already being loaded, remove either the LoadModule in httpd.conf or remove the php.conf file in /etc/httpd/conf.d.

8. Now you should have images support for PHP!


[edit] Tips

* If any compilation fails and you have to reconfigure, make sure you do "make clean" before doing a make.
* Listen to some relaxing music while doing this, because wading through unclear documentation for your desired setup could be very frustrating. I suggest Nick Drake's Pink Moon album to help things along.
* Visit PHP.net's Image Functions for the most recent URLs for the libraries and instructions for configuring images and more information.

mod_rewrite

URL Rewriting

The Apache server’s mod_rewrite module gives you the ability to transparently redirect one URL to another, without the user’s knowledge. This opens up all sorts of possibilities, from simply redirecting old URLs to new addresses, to cleaning up the ‘dirty’ URLs coming from a poor publishing system — giving you URLs that are friendlier to both readers and search engines.

An Introduction to Rewriting

Readable URLs are nice. A well designed website will have a logical file system layout, with smart folder and file names, and as many implementation details left out as possible. In the most well designed sites, readers can guess at filenames with a high level of success.

However, there are some cases when the best possible information design can’t stop your site’s URLs from being nigh-on impossible to use. For instance, you may be using a Content Management System that serves out URLs that look something like

http://www.example.com/viewcatalog.asp?category=hats&prodID=53

This is a horrible URL, but it and its brethren are becoming increasingly prevalent in these days of dynamically-generated pages. There are a number of problems with an URL of this kind:

*

It exposes the underlying technology of the website (in this case ASP). This can give potential hackers clues as to what type of data they should send along with the query string to perform a ‘front-door’ attack on the site. Information like this shouldn’t be given away if you can help it.

Even if you’re not overly concerned with the security of your site, the technology you’re using is at best irrelevant — and at worst a source of confusion — to your readers, so it should be hidden from them if possible.

Also, if at some point in the future you decide to change the language that your site is based on (to » PHP, for instance); all your old URLs will stop working. This is a pretty serious problem, as anyone who has tackled a full-on site rewrite will attest.
* The URL is littered with awkward punctuation, like the question mark and ampersand. Those & characters, in particular, are problematic because if another webmaster links to this page using that URL, the un-escaped ampersands will mess up their XHTML conformance. They will have to laboriously replace all the ampersands with & character entities, which is often forgotten.
* Some search engines won’t index pages which they think are generated dynamically. They’ll see that question mark in the URL and just turn their asses around.

Luckily, using rewriting, we can clean up this URL to something far more manageable. For example, we could map it to

http://www.example.com/catalog/hats/53/

Much better. This URL is more logical, readable and memorable, and will be picked up by all search engines. The faux-directories are short and descriptive. Importantly, it looks more permanent.

To use mod_rewrite, you supply it with the link text you want the server to match, and the real URLs that these URLs will be redirected to. The URLs to be matched can be straight file addresses, which will match one file, or they can be regular expressions, which will match many files.
Basic Rewriting

Some servers will not have » mod_rewrite enabled by default. As long as the » module is present in the installation, you can enable it simply by starting a .htaccess file with the command

RewriteEngine on

Put this .htaccess file in your root so that rewriting is enabled throughout your site. You only need to write this line once per .htaccess file.
Basic Redirects

We’ll start off with a straight redirect; as if you had moved a file to a new location and want all links to the old location to be forwarded to the new location. Though you shouldn’t really ever » move a file once it has been placed on the web; at least when you simply have to, you can do your best to stop any old links from breaking.

RewriteEngine on
RewriteRule ^old\.html$ new.html

Though this is the simplest example possible, it may throw a few people off. The structure of the ‘old’ URL is the only difficult part in this RewriteRule. There are three special characters in there.

* The caret, ^, signifies the start of an URL, under the current directory. This directory is whatever directory the .htaccess file is in. You’ll start almost all matches with a caret.
* The dollar sign, $, signifies the end of the string to be matched. You should add this in to stop your rules matching the first part of longer URLs.
* The period or dot before the file extension is a special character in regular expressions, and would mean something special if we didn’t escape it with the backslash, which tells Apache to treat it as a normal character.

So, this rule will make your server transparently redirect from old.html to the new.html page. Your reader will have no idea that it happened, and it’s pretty much instantaneous.
Forcing New Requests

Sometimes you do want your readers to know a redirect has occurred, and can do this by forcing a new HTTP request for the new page. This will make the browser load up the new page as if it was the page originally requested, and the location bar will change to show the URL of the new page. All you need to do is turn on the [R] flag, by appending it to the rule:

RewriteRule ^old\.html$ new.html [R]

Using Regular Expressions

Now we get on to the really useful stuff. The power of mod_rewrite comes at the expense of complexity. If this is your first encounter with regular expressions, you may find them to be a tough nut to crack, but the options they afford you are well worth the slog. I’ll be providing plenty of examples to guide you through the basics here.

Using regular expressions you can have your rules matching a set of URLs at a time, and mass-redirect them to their actual pages. Take this rule;

RewriteRule ^products/([0-9][0-9])/$ /productinfo.php?prodID=$1

This will match any URLs that start with ‘products/’, followed by any two digits, followed by a forward slash. For example, this rule will match an URL like products/12/ or products/99/, and redirect it to the PHP page.

The parts in square brackets are called ranges. In this case we’re allowing anything in the range 0-9, which is any digit. Other ranges would be [A-Z], which is any uppercase letter; [a-z], any lowercase letter; and [A-Za-z], any letter in either case.

We have encased the regular expression part of the URL in parentheses, because we want to store whatever value was found here for later use. In this case we’re sending this value to a PHP page as an argument. Once we have a value in parentheses we can use it through what’s called a back-reference. Each of the parts you’ve placed in parentheses are given an index, starting with one. So, the first back-reference is $1, the third is $3 etc.

Thus, once the redirect is done, the page loaded in the readers’ browser will be something like productinfo.php?prodID=12 or something similar. Of course, we’re keeping this true URL secret from the reader, because it likely ain’t the prettiest thing they’ll see all day.
Multiple Redirects

If your site visitor had entered something like products/12, the rule above won’t do a redirect, as the slash at the end is missing. To promote good URL writing, we’ll take care of this by doing a direct redirect to the same URL with the slash appended.

RewriteRule ^products/([0-9][0-9])$ /products/$1/ [R]

Multiple redirects in the same .htaccess file can be applied in sequence, which is what we’re doing here. This rule is added before the one we did above, like so:

RewriteRule ^products/([0-9][0-9])$ /products/$1/ [R]
RewriteRule ^products/([0-9][0-9])/$ /productinfo.php?prodID=$1

Thus, if the user types in the URL products/12, our first rule kicks in, rewriting the URL to include the trailing slash, and doing a new request for products/12/ so the user can see that we likes our trailing slashes around here. Then the second rule has something to match, and transparently redirects this URL to productinfo.php?prodID=12. Slick.
Match Modifiers

You can expand your regular expression patterns by adding some modifier characters, which allow you to match URLs with an indefinite number of characters. In our examples above, we were only allowing two numbers after products. This isn’t the most expandable solution, as if the shop ever grew beyond these initial confines of 99 products and created the URL productinfo.php?prodID=100, our rules would cease to match this URL.

So, instead of hard-coding a set number of digits to look for, we’ll work in some room to grow by allowing any number of characters to be entered. The rule below does just that:

RewriteRule ^products/([0-9]+)$ /products/$1/ [R]

Note the plus sign (+) that has snuck in there. This modifier changes whatever comes directly before it, by saying ‘one or more of the preceding character or range.’ In this case it means that the rule will match any URL that starts with products/ and ends with at least one digit. So this’ll match both products/1 and products/1000.

Other match modifiers that can be used in the same way are the asterisk, *, which means ‘zero or more of the preceding character or range’, and the question mark, ?, which means ‘zero or only one of the preceding character or range.’
Adding Guessable URLs

Using these simple commands you can set up a slew of ‘shortcut URLs’ that you think visitors will likely try to enter to get to pages they know exist on your site. For example, I’d imagine a lot of visitors try jumping straight into our stylesheets section by typing the URL http://www.yourhtmlsource.com/css/. We can catch these cases, and hopefully alert the reader to the correct address by updating their location bar once the redirect is done with these lines:

RewriteRule ^css(/)?$ /stylesheets/ [R]

The simple regular expression in this rule allows it to match the css URL with or without a trailing slash. The question mark means ‘zero or one of the preceding character or range’ — in other words either yourhtmlsource.com/css or yourhtmlsource.com/css/ will both be taken care of by this one rule.

This approach means less confusing 404 errors for your readers, and a site that seems to run a whole lot smoother all ’round.

Downloading files Using PHP

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