Noupe.com has posted a new guide to getting started with regular expressions in PHP. Regular expressions can be one of the most powerful tools at your disposal when used correctly.

The main purpose of regular expressions, also called regex or regexp, is to efficiently search for patterns in a given text. These search patterns are written using a special format which a regular expression parser understands.

They cover some of the basics - special characters and some examples using them (like $, d, and *) - before getting into the preg_* functions in PHP. They also include some handy examples to validate things like US zip codes, email format validation and removing duplicated words in a section of text.

On CatsWhoCode.com today there's a new article listing fifteen handy regular expressions you might find useful in your day-to-day development (as well as a brief introduction to what regular expressions are).

Regular expressions are a very useful tool for developers. They allow to find, identify or replace text, words or any kind of characters. In this article, I have compiled 15+ extremely useful regular expressions that any web developer should have in his toolkit.

Here's just some of the expressions on the list:

• Enlight a word from a text
• Remove repeated words (case insensitive)
• Matching a XML/HTML tag
• Matching hexadecimal color values
• Parsing Apache logs

On PHPBuilder.com there's a new installment in their "ABCs of PHP" series posted - a look at working with regular expressions.

We've looked at strings, and numbers and all sorts of types of data, but we've not yet seen how to do something really important, and that's to look for and pull interesting parts out of the data we have, to do that where going to use some magic from the Perl world called "Regular Expressions".

They start simple by looking at what regular expressions are and how they work to match portions (or all) of a string. Thankfully, they chose to go with the preg_* functions (like preg_match and friends) for their matching over the ereg_* functions. They talk about direct matches, wildcards and partial matching.

On DevShed today there's a new tutorial looking at a method for protecting your application (your MySQL/PHP application) forms from malicious attacks like SQL injections.

It has been known for a while that if a form is unsecured, malicious code in the form of MySQL injection will be initiated to attack the site. HTML forms such as drop down menus, search box, check boxes are all susceptible entry points of this type of abuse. This article will explain what happens in this kind of attack, and how to prevent it.

They compare unvalidated input with a validated method that uses some simple checks to ensure that the given content is numeric and regular expressions to match text.

On the SocialGeek blog there's a recent post that looks at making stubs for your URLs, making them easier to read and remember.

This is where the fun begins of course. How many times have you been confronted with someone sending you an indecipherable, thus untrustworthy link? Right, so we agree that for a user, it is important to have a clean URL that is readable and includes the title of the page or (at least) some description related to the content. Slug time!

They explain what slugs are (and how they're useful for users) as well as how to convert a title into a "slugged" string by replacing anything that's not an A-Z or 0-9 character to remove the less URL friendly characters.

PHPFreaks.com has posted part one of a series of tutorials covering some of the basics of one of the more powerful features in any language - regular expression support.

Regular expressions (which will now be referred to as "regexes") are basically pattern matching inside of text. They use special syntax and concepts in order to obtain information from a string. Many programming languages have some sort of support for regexes, because of the sheer usefulness of them. Not only can patterns be used to validate that a certain pattern exists in a string, but they can also be used to physically extract matched portions and make them usable in your PHP code.

They look at some of the basics - example structure, metacharacters, greediness as well as include a brief comparison of PCRE versus POSIX versions. The tutorial is peppered with plenty of example expressions too, giving you a good idea of what you're working towards knowing.

DevShed has a new article posted today spotlighting the preg_replace_callback function and how it compares to a normal preg_replace call in some everyday situations.

The preg_replace_callback() function is an under-utilized and little documented PHP function that you will find useful and powerful in many situations, especially if you have been actively using preg_match() and preg_replace(). In this article, you will find a clear explanation of what this function does, together with carefully crafted examples that illustrate some of its uses.

They show how to use it in a few different situations - inside conditional statements, for loops, in a sequence and inside of a class to manipulate matched strings with external functions (both native to PHP and user-defined).

Ken Guest, in a response to another post from a different blogger, has posted some of his own validation replacements for the regular expression method the other blogger chose.

I've noticed that Richard Heyes, who professes himself to be a php guru, deleted my comment on his "Some common regular expressions" posting which simply pointed out his expressions didn't quite do the job and suggested a few PEAR packages that should be used instead of the expressions that he proffered

His examples have the benefit of what he calls "defense in depth" - the functionality to catch a bit more than just a regular expression can alone. His examples include PEAR_Validate for email addresses, Net_CheckIP2 for IP addresses and the Validate_UK package for the sort code and telephone numbers.

Eirik Hoem has pointed out an online tool a coworker shared with him to work with regular expressions for multiple languages.

A coworker of mine has been working on a ajax enabled regex tool which lets you evaluate regex expressions in several languages (including PHP PCRE and PHP POSIX) with instant results.

The tool lets you put in the string you want to match again and the pattern you want to match with. The results are automatically populated below it, making it easy to fine-tune your expression to only what you want.

On the Developer Tutorials blog, Akash Mehta offers some suggestions of resources and methods for learning how to use regular expressions in your PHP applications.

When it comes to quickly dealing with large blocks of data, batch processing operations or screen scraping, regular expressions are often the most effective solution. There's just one problem, though - learning them can be as hard as learning a new language altogether. Here's how to get off to a flying start.

He points you first in the direction of the preg_* functions then towards a few examples (like with mod_rewrite) and tools to help you understand how things match, like the regex tested extension for firefox and the regular expression cheat sheet on AddedBytes.com.