PHP Regular Expressions
PHP Regular Expressions
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.
Regular Expression Syntax
In PHP, regular expressions are strings composed of delimiters, a pattern and optional modifiers.
Syntax
"/pattern/modifiers"- delimiters - charcters that enclose the pattern (e.g.
/) - pattern - the character sequence to search for
- modifiers - how the search is performed (e.g.
iindicates a case-insensitive search)
Look at the following regular expression:
$exp = "/w3schools/i";
Here, / is the delimiter, w3schools is the pattern to search 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 Modifiers
The modifiers specify how the search is performed.
| Modifier | Description | Try it |
|---|---|---|
| i | Performs a case-insensitive search | Try it » |
| 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) | Try it » |
| u | Enables correct matching of UTF-8 encoded patterns |
Regular Expression Patterns
Brackets are used to find a range of characters:
| Expression | Description | Try it |
|---|---|---|
| [abc] | Find one or many of the characters inside the brackets | Try it » |
| [^abc] | Find any character NOT between the brackets | Try it » |
| [a-z] | Find any character alphabetically between two letters | Try it » |
| [A-z] | Find any character alphabetically between a specified upper-case letter and a specified lower-case letter | Try it » |
| [A-Z] | Find any character alphabetically between two upper-case letters. | Try it » |
| [123] | Find one or many of the digits inside the brackets | Try it » |
| [0-5] | Find any digits between the two numbers | Try it » |
| [0-9] | Find any digits | Try it » |
Metacharacters
Metacharacters are characters with a special meaning:
| Metacharacter | Description | Try it |
|---|---|---|
| | | Find a match for any one of the patterns separated by | as in: cat|dog|fish | Try it » |
| . | Find any character | Try it » |
| ^ | Finds a match as the beginning of a string as in: ^Hello | Try it » |
| $ | Finds a match at the end of the string as in: World$ | Try it » |
| \d | Find any digits | Try it » |
| \D | Find any non-digits | Try it » |
| \s | Find any whitespace character | Try it » |
| \S | Find any non-whitespace character | Try it » |
| \w | Find any alphabetical letter (a to Z) and digit (0 to 9) | Try it » |
| \W | Find any non-alphabetical and non-digit character | Try it » |
| \b | Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b | Try it » |
| \uxxxx | Find the Unicode character specified by the hexadecimal number xxxx | Try it » |
Quantifiers
Quantifiers define quantities:
| Quantifier | Description | Try it |
|---|---|---|
| n+ | Matches any string that contains at least one n | Try it » |
| 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 | Try it » |
| n{2, 5} | Matches any string that contains a sequence of at least 2, but not more that 5 n's | Try it » |
| n{3,} | Matches any string that contains a sequence of at least 3 n's | Try it » |
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 = '/\?+/';
