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Getting Started with PHP Regular Expressions

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Jun 23, 2016 pm 02:30 PM

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1. What are Regular Expressions

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.

Regular expressions are originating from Unix systems, where a program was designed, called grep, to help 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 be solved thanks to regular expressions. The syntax doesn’t always seems straightforward at first, but once you learn it, you’ll realize that you can do pretty complex searches easily, just by typing in a few characters and you’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 are two 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 is deprecated, but let’s have 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 you 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 different parentheses.

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

Now, let’s have 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 will match the preceding pattern zero or one times
+	It will match the preceding pattern one or more times
*	It will 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 is 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 is escaped using the simple slash \. Example: ‘/he\/llo/’

To have a brief understanding how these operators are used, let’s have a look at a few examples:

Example Description

‘/hello/’	It will match the word hello
‘/^hello/’	It will match hello at the start of a string. Possible matches are hello orhelloworld, but not worldhello
‘/hello$/’	It will match hello at the end of a string.
‘/he.o/’	It will match any character between he and o. Possible matches are heloor heyo, but not hello
‘/he?llo/’	It will match either llo or hello
‘/hello+/’	It will match hello on or more time. E.g. hello or hellohello
‘/he*llo/’	Matches llo, hello or hehello, but not hellooo
‘/hello\|world/’	It will either match the word hello or world
‘/(A-Z)/’	Using it with the hyphen character, this pattern will match every uppercase character from A to Z. E.g. A, B, C…
‘/[abc]/’	It will match any single character a, b or c
‘/abc{1}/’	Matches precisely one c character after the characters ab. E.g. matchesabc, but not abcc
‘/abc{1,}/’	Matches one or more c character after the characters ab. E.g. matches abcor abcc
‘/abc{2,4}/’	Matches between two and four c character after the characters ab. E.g. matches abcc, abccc or abcccc, but not abc

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

The regex modifiers are placed after the pattern, like this ‘/hello/i’ and they consists of single letters such as i which marks a pattern case insensitive or x which ignores white-space characters. For a full list of modifiers please 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 have 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 two most commonly used functions are preg_match and preg_replace.

Let’s begin by creating a test string on which we will perform our regular expression searches. The classical hello world should do it.