Getting Started with PHP Regular Expressions

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1. What is Regular Expressions

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

Regular expressions is originating from Unix systems, where a program is designed, called grep, to-help the users work with strings and manipulate text. By following a few basic rules, one can create very complex search patterns.

As an example, let's say you ' re given the task to check wether an e-mail or a telephone number has the correct form. Using a few simple commands these problems can easily is solved thanks to regular expressions. The syntax doesn ' t always seems straightforward in first, but once you learn it, and you'll realize that's can do pretty com Plex searches easily, just by typing in a few characters and I ' ll approach problems from a different perspective.

2. Perl Compatible Regular Expressions

PHP has implemented quite a few regex functions which uses different parsing engines. There is major parser in PHP. One called POSIX and the other PCRE or Perl Compatible Regular Expression.

The PHP function prefix for POSIX is ereg_. Since the release of PHP 5.3 This engine was deprecated, but let's has a look at the more optimal and faster PCRE engine.

In PHP every PCRE function starts with preg_ such as Preg_match or preg_replace. You can read the full function list in PHP ' s documentation.

3. Basic Syntax

To use regular expressions first your need to learn the syntax. This syntax consists in a series of letters, numbers, dots, hyphens and special signs, which we can group together using D Ifferent parentheses.

In PHP every regular expression pattern is defined as a string using the Perl format. In Perl, the a regular expression pattern is written between forward slashes, such as/hello/. In PHP this would become a string, '/hello/'.

Now, let's has a look at some operators, the basic building blocks of regular expressions

Operator Description

^	The circumflex symbol marks the beginning of a pattern, although in some cases it can be omitted
$	Same as with the circumflex symbol, the dollar sign marks the end of a search pattern
.	The period matches any single character
?	It'll match the preceding pattern zero or one times
+	It'll match the preceding pattern one or more times
*	It'll match the preceding pattern zero or more times
\|	Boolean OR
-	Matches a range of elements
()	Groups a different pattern elements together
[]	Matches any single character between the square brackets
{min, max}	It's used to match exact character counts
\d	Matches any single digit
\d	Matches any single non digit caharcter
\w	Matches any alpha numeric character including underscore (_)
\w	Matches any non alpha numeric character excluding the underscore character
\s	Matches whitespace character

As an addition in PHP the forward slash character are escaped using the simple slash \. Example: '/he\/llo/'

To has a brief understanding how these operators is used, let ' s has a look at a few examples:

Example Description

'/hello/'	It'll match the word hello
'/^hello/'	It would match hello at the start of a string. Possible matches is Hello Orhelloworld, but not Worldhello
'/hello$/'	It would match hello at the end of a string.
'/he.o/'	It'll match any character between he and O. Possible matches is Heloor heyo, but not hello
'/he?llo/'	It'll match either Llo or hello
'/hello+/'	It'll match hello on or more time. e.g. hello or Hellohello
'/he*llo/'	Matches Llo, hello or Hehello, but not hellooo
'/hello\|world/'	It'll either match the word hello or world
'/(A-Z)/'	Using it with the hyphen character, this pattern would match every uppercase character from a to Z. e.g. a, B, C ...
'/[abc]/'	It would match any single character A, B or C
'/abc{1}/'	Matches precisely one C character after the characters ab e.g. MATCHESABC, and not ABCC
'/abc{1,}/'	Matches one or more C character after the characters ab e.g. Matches Abcor ABCC
'/abc{2,4}/'	Matches between and four C character after the characters ab e.g. Matches ABCC, ABCCC or ABCCCC, but not ABC

Besides operators, there is regular expression modifiers, which can globally alter the behavior of search patterns.

The regex modifiers is placed after the pattern, such as this '/hello/i ' and they consists of a single letters such as I which Marks a pattern case insensitive or x which ignores white-space characters. For a full list of modifiers visit PHP ' s online documentation.

The real power of regular expressions relies in combining these operators and modifiers, therefore creating rather complex Search patterns.

4. Using Regex in PHP

In PHP we had a total of nine PCRE functions which we can use. Here's the list:

Preg_filter? Performs a regular expression search and replace Preg_grep? Returns array entries that match a pattern preg_last_error? Returns the error code of the last PCRE regex execution Preg_match? Perform a regular expression match preg_match_all? Perform a global regular expression match preg_quote? Quote Regular expression characters preg_replace? Perform a regular expression search and replace Preg_replace_callback? Perform a regular expression search and replace using a callback preg_split? Split string by a regular expression

The most commonly used functions is preg_match and preg_replace.

Let's begin by creating a test string on which we'll perform our regular expression searches. The classical Hello world should do it.