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.
In the example above, / is the delimiter, w3schools 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 preg_match[], preg_match_all[] and preg_replace[] functions are some of the most commonly used ones:
| 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 "w3schools" in a string:
Try it Yourself »
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:
Try it Yourself »
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 Microsoft with W3Schools in a string:
Try it Yourself »
Regular Expression Modifiers
Modifiers can change how a search is performed.
| i | Performs a case-insensitive search |
| m | Performs a multiline search [patterns that search for the beginning or end of a string will 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:
| [abc] | Find one character from the options between the brackets |
| [^abc] | Find any character NOT between the brackets |
| [0-9] | Find one character from the range 0 to 9 |
Metacharacters
Metacharacters are characters with a special meaning:
| | | Find a match for any one of the patterns separated by | as in: cat|dog|fish |
| . | Find just one instance of 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 a digit |
| \s | Find a whitespace 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:
| 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{x} | Matches any string that contains a sequence of X n's |
| n{x,y} | Matches any string that contains a sequence of X to Y n's |
| n{x,} | Matches any string that contains a sequence of at least X 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:
Try it Yourself »
Complete RegExp Reference
For a complete reference, go to our Complete PHP Regular Expression Reference.
The reference contains descriptions and examples of all Regular Expression functions.