Regular expressions are used to find specific patterns, to replace characters in a string using specific patterns, or mostly used to validate inputs to match specific pattern.
What is a Regular Expression?
A regular expression is a sequence or pattern of characters and symbols used to find, find and replace characters and validate a part of string. We will go through few simple examples to see how regular expressions play role in finding specific patterns , replace them or just validate a string.
Regular expressions are created using a specific syntax, Like /php/ where "/" at the start and end indicates that its a regular expression "php" indicates the pattern. This regular expression when used whether to search or replace it will look for exact word "php" in a string. Following are mostly used PHP functions for regular expressions:
- preg_match($pattern, $subject, &$matches, $flags, $offset): For a regular expression search, return true if matched and false if no match found.
- preg_match_all($pattern, $subject, &$matches, $flags, $offset): For a global regular expression search, the difference if the first match is found it keeps searching for provided pattern until the end of string.
- preg_replace($pattern, $replacement, $subject, $limit, &$count): For a regular expression search and replace for a provided pattern. Pattern can be passed as a string or array of strings.
- preg_replace_callback($pattern, $callback, $subject, $limit, &$count, $flags): Performs a regular expression search and replace using a callback function.
For detailed usage of these function refer to PHP's PCRE Functions. Let's assume, we need to search for a word "php" in this string "this is a string in php", We will do it like so:
preg_match("/php/", "this is a string in php");
Regular Expressions Examples:
| Expression |
Definition |
Example |
Sample |
| ^ |
Start of string |
/^abc/ |
abcword |
| $ |
End of string |
/abc$/ |
wordabc |
| \d |
Any digit |
/word\d/ |
word5 |
| \w |
Any alphanumeric or underscore |
/123\w/ |
123Z |
| [0-9] |
Numbers from 0 to 9 |
/word[0-9]/ |
word1 |
| [a-z] |
Letters from a to z |
/123[a-z]/ |
123a |
| [A-Z] |
Letters from A to Z |
/123[A-Z]/ |
123W |
| + |
Minimum one occurrence |
/\d+/ |
123456 |
| * |
Zero or more occurrences |
/\d+A*/ |
123AAA or 123 |
| ? |
Zero or one occurrence |
/\d+ab?c/ |
123abc or 123ac |
| . |
Any character that is not a line break |
/x.z/ |
xyz |
| {3} |
Exactly three occurrences |
/a{3}/ |
aaa |
| {2,4} |
Minimum two and maximum four occurrences |
/a{2,4}/ |
aa |
| {2,} |
Two or more occurrences |
/\w{2,}/ |
word_word |
Mostly Used Regular Expressions:
| Verify only digits with regular expression: |
| Expression |
Sample |
Result |
| /^[0-9]+$/ |
123456 |
Pass |
| 123abc |
Fail |
|
| Verify only alphabets with regular expression: |
| Expression |
Sample |
Result |
| /^[a-zA-Z]+$/ |
abcXYZ |
Pass |
| abcXYZ12 |
Fail |
|
| Verify email address with regular expression: |
| Expression |
Sample |
Result |
| /^[\w\-\.]+\@[a-zA-Z0-9]+\.[a-zA-Z]{2,4}$/ |
[email protected] |
Pass |
| user@domain |
Fail |
| userdomain.com |
Fail |
|
| Verify url with regular expression: |
| Expression |
Sample |
Result |
| /^((https?|ftp):\/\/)(www\.)?[\w\-]+\.[a-z]{2,4}\/?$/ |
https://domain.com |
Pass |
| http://domain.com |
Pass |
| http://www.domain.com |
Pass |
| www.domain.com |
Fail |
|
