PHP Regular Expressions (RegEx)
What is a Regular Expression?
A regular expression is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for.
A regular expression can be a single character, or a more complicated pattern.
Regular expressions can be used to perform all types of text search and text replace operations.
Syntax
In PHP, regular expressions are strings composed of delimiters, a pattern and optional modifiers.
$exp = "/scoolio.academy/i";
In the example above, / is the delimiter, scoolio.academy is the pattern that is being searched for, and i is a modifier that makes the search case-insensitive.
The delimiter can be any character that is not a letter, number, backslash or space. The most common delimiter is the forward slash (/), but when your pattern contains forward slashes it is convenient to choose other delimiters such as # or ~.
Regular Expression Functions
PHP provides a variety of functions that allow you to use regular expressions.
The most common functions are:
| Function | Description |
|---|---|
| preg_match() | Returns 1 if the pattern was found in the string and 0 if not |
| preg_match_all() | Returns the number of times the pattern was found in the string, which may also be 0 |
| preg_replace() | Returns a new string where matched patterns have been replaced with another string |
Using preg_match()
The preg_match() function will tell you whether a string contains matches of a pattern.
Example
Use a regular expression to do a case-insensitive search for "scoolio.academy" in a string:
$str = "Visit scoolio.academy";
$pattern = "/scoolio.academy/i";
echo preg_match($pattern, $str);
Using preg_match_all()
The preg_match_all() function will tell you how many matches were found for a pattern in a string.
Example
Use a regular expression to do a case-insensitive count of the number of occurrences of "ain" in a string:
$str = "The rain in SPAIN falls mainly on the plains.";
$pattern = "/ain/i";
echo preg_match_all($pattern, $str);
Using preg_replace()
The preg_replace() function will replace all of the matches of the pattern in a string with another string.
Example
Use a case-insensitive regular expression to replace Google with Scoolio in a string:
$str = "Visit Google!";
$pattern = "/google/i";
echo preg_replace($pattern, "Scoolio", $str);
Regular Expression Modifiers
Modifiers can change how a search is performed.
| Modifier | Description |
|---|---|
| i | Performs a case-insensitive search |
| m | Performs a multiline search (patterns that search for a match at the beginning or end of a string will now match the beginning or end of each line) |
| u | Enables correct matching of UTF-8 encoded patterns |
Regular Expression Patterns
Brackets are used to find a range of characters:
| Expression | Description |
|---|---|
| [abc] | Find one or many of the characters inside the brackets |
| [^abc] | Find any character NOT between the brackets |
| [a-z] | Find any character alphabetically between two letters |
| [A-z] | Find any character alphabetically between a specified upper-case letter and a specified lower-case letter |
| [A-Z] | Find any character alphabetically between two upper-case letters. |
| [123] | Find one or many of the digits inside the brackets |
| [0-5] | Find any digits between the two numbers |
| [0-9] | Find any digits |
Metacharacters
Metacharacters are characters with a special meaning:
| Metacharacter | Description |
|---|---|
| | | Find a match for any one of the patterns separated by | as in: cat|dog|fish |
| . | Find any character |
| ^ | Finds a match as the beginning of a string as in: ^Hello |
| $ | Finds a match at the end of the string as in: World$ |
| \d | Find any digits |
| \D | Find any non-digits |
| \s | Find any whitespace character |
| \S | Find any non-whitespace character |
| \w | Find any alphabetical letter (a to Z) and digit (0 to 9) |
| \W | Find any non-alphabetical and non-digit character |
| \b | Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b |
| \uxxxx | Find the Unicode character specified by the hexadecimal number xxxx |
Quantifiers
Quantifiers define quantities:
| Quantifier | Description |
|---|---|
| n+ | Matches any string that contains at least one n |
| n* | Matches any string that contains zero or more occurrences of n |
| n? | Matches any string that contains zero or one occurrences of n |
| n{3} | Matches any string that contains a sequence of 3 n's |
| n{2, 5} | Matches any string that contains a sequence of at least 2, but not more that 5 n's |
| n{3,} | Matches any string that contains a sequence of at least 3 n's |
Note: If your expression needs to search for one of the special characters you can use a backslash ( \ ) to escape them. For example, to search for one or more question marks you can use the following expression: $pattern = '/\?+/';
Grouping
You can use parentheses ( ) to apply quantifiers to entire patterns. They also can be used to select parts of the pattern to be used as a match.
Example
Use grouping to search for the word "banana" by looking for ba followed by two instances of na:
$str = "Apples and bananas.";
$pattern = "/ba(na){2}/i";
echo preg_match($pattern, $str);