What are regular expressions?

A regular expression is a simple programming language (for lack of a better term) which allows a programmer to search through text for a specific pattern.

For example, if I want to make sure a string contains a valid email address. I might need to match the following items in the string...

• It starts with a series of characters (letters or numbers) which may or may not include a period or underscore
• It will contain an "@" symbol
• It will then contain another series of characters (possibly including a period or underscore).
• It will end in a valid top level domain (.com, .net, .org or .edu for example)

A regular expression to verify this might look something like:

"/[\w\.]{3,}@[\w\.]{1,}\.(net|com|org|edu)/"

PHP contains numerous functions to use with regular expressions which usually boil down to two groups of functions: ereg, and preg.

The differences between ereg and preg are very small and for this class we will be using the preg group of functions.

To check a string for a particular pattern in PHP, we can use the preg_match function.

  $email = "REDACTED";
  if(preg_match(\"/[\w\.]{3,}@[\w\.]{1,}\.(net|com|org|edu)/\",$email)) {
    print \"match\";
  }
  else {
    print \"no match\";
  }

We'll get into what this means later

Many many uses...

As we see above, regular expressions can come in very handy for verifying data matches a certain pattern, however that is not the limit of regular expressions. We can use regular expressions to extract data from something.

For example, we could use a regular expression to get the information from a meta tag off an html document, or we could use a regular expression to rip through a log file on a web server and show us everytime a particular page was requested.