The ultimate regular expressions guide and cheat sheet. The most concise and complete guide on regular expressions on the Internet. The ultimate regex cheat sheet. (BRE) and PCRE (Perl Compatible Regular Expressions). Of these two PCRE is the newest one, and it was designed to be more powerful and flexible than BRE. Regular expression tester with syntax highlighting, PHP / PCRE & JS Support, contextual help, cheat sheet, reference, and searchable community patterns. RegExr is an online tool to learn, build, & test Regular Expressions (RegEx / RegExp). PCRE (Perl Compatible Regular Expressions) is a C library implementing regex. It was written in 1997 when Perl was the de-facto choice for complex text processing tasks. The syntax for patterns used in PCRE closely resembles Perl. Beginning of String or End of Previous Match.NET, Java, PCRE (C, PHP, R), Perl, Ruby b Word boundary Most engines: position where one side only is an ASCII letter, digit or underscore.
Regular expressions are a very useful tool for developers. They allow to find, identify or replace a word, character or any kind of string. This tutorial will teach you how to master PHP regexp and show you extremely useful, ready-to-use PHP regular expressions that any web developer should have in his toolkit.
Getting Started With Regular Expressions
For many beginners, regular expressions seem to be hard to learn and use. In fact, they’re far less hard than you may think. Before we dive deep inside regexp with useful and reusable codes, let’s quickly see the basics of PCRE regex patterns:
Regular Expressions Syntax
A regular expression (regex or regexp for short) is a special text string for describing a search pattern. A regex pattern matches a target string. The following table describes most common regex:
| Regular Expression | Will match… |
|---|---|
| foo | The string “foo” |
| ^foo | “foo” at the start of a string |
| foo$ | “foo” at the end of a string |
| ^foo$ | “foo” when it is alone on a string |
| [abc] | a, b, or c |
| [a-z] | Any lowercase letter |
| [^A-Z] | Any character that is not a uppercase letter |
| (gif|jpg) | Matches either “gif” or “jpg” |
| [a-z]+ | One or more lowercase letters |
| [0-9.-] | Any number, dot, or minus sign |
| ^[a-zA-Z0-9_]{1,}$ | Any word of at least one letter, number or _ |
| ([wx])([yz]) | wy, wz, xy, or xz |
| [^A-Za-z0-9] | Any symbol (not a number or a letter) |
| ([A-Z]{3}|[0-9]{4}) | Matches three letters or four numbers |
PHP Regular Expression Functions
PHP has many useful functions to work with regular expressions. Here is a quick cheat sheet of the main PHP regex functions. Remember that all of them are case sensitive.
For more information about the native functions for PHP regular expressions, have a look at the manual.
| Function | Description |
|---|---|
| preg_match() | The preg_match() function searches string for pattern, returning true if pattern exists, and false otherwise. |
| preg_match_all() | The preg_match_all() function matches all occurrences of pattern in string. Useful for search and replace. |
| preg_replace() | The preg_replace() function operates just like ereg_replace(), except that regular expressions can be used in the pattern and replacement input parameters. |
| preg_split() | Preg Split (preg_split()) operates exactly like the split() function, except that regular expressions are accepted as input parameters. |
| preg_grep() | The preg_grep() function searches all elements of input_array, returning all elements matching the regex pattern within a string. |
| preg_ quote() | Quote regular expression characters |
Validate a Domain Name
Case sensitive regex to verify if a string is a valid domain name. This is very useful when validating web forms.
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Enlight a Word From a Text
This very useful regular expression will find a specific word in a string and enlight it. Extremely useful for search results. Remember that it’s case sensitive.
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Enlight Search Results in Your WordPress Blog
The previous code snippet can be very handy when it comes to displaying search results. If your website is powered by WordPress, here is a more specific snippet that will search and replace a text by the same text within an HTML tag that you can style later, using CSS.
Open your search.php file and find the the_title() function. Replace it with the following:
Now, just before the modified line, add this code:
Save the search.php file and open style.css. Append the following line to it:
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Get All Images From a HTML Document
If you ever wanted to be able to get all images form a webpage, this code is a must have for you. You should easily create an image downloader using the power of cURL.
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Remove Repeated Words (Case Insensitive)
Often repeating words while typing? This handy case insensitive PCRE regex will be very helpful.
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Remove Repeated Punctuation
Same php regex as above, but this one will look for repeated punctuation within a string. Goodbye multiple commas!
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Match a XML/HTML Tag
This simple function takes two arguments: The first is the tag you’d like to match, and the second is the variable containing the XML or HTML. Once again, this can be very powerful used along with cURL.
Match an HTML/XML Tag With a Specific Attribute Value
This function is very similar to the previous one, but it allow you to match a tag having a specific attribute. For example, you could easily match <div>.
Match Hexadecimal Color Values
Another interesting tool for web developers! It allows you to match/validate a hexadecimal color value.
Find Page Title
This handy code snippet will find and print the text within the <title> and </title> tags of a HTML page.
Parse Apache Logs
Most websites are running on the Apache webserver. If your website does, you can easily use PHP and regular expressions to parse Apache logs.
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Replace Double Quotes by Smart Quotes
If you’re a typography lover, you’ll probably love this regex pattern which allow you to replace double quotes by smart quotes. A similar regular expression is used by WordPress to make the content more beautiful.
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Check Password Complexity
Perl Regex Cheat Sheet
This regular expression will tests if the input consists of 6 or more letters, digits, underscores, and hyphens.
The input must contain at least one uppercase letter, one lowercase letter and one digit.
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WordPress: Using Regexp to Retrieve Images From a Post
As I know many of you are WordPress users, you’ll probably enjoy that code which allows you to retrieve all images from post content and display it.
To use this code on your blog, simply paste the following code on one of your theme files.
Generate Emoticons Automatically
Another function used by WordPress. This one allow you to automatically replace an emoticon symbol by an image.
PCRE (Perl Compatible Regular Expressions) is a C library implementing regex. It was written in 1997 when Perl was the de-facto choice for complex text processing tasks. The syntax for patterns used in PCRE closely resembles Perl. PCRE syntax is being used in many big projects called flavors – .NET, Java, JavaScript, XRegExp, Perl, PCRE, Python, and Ruby, and the programming languages C#, Java, JavaScript, Perl, PHP, Python, Ruby, and VB.NET.
PCRE’s syntax is much more powerful and flexible than either of the POSIX regular expression flavors and than that of many other regular-expression libraries.
we’re focused on PRCE mostly unless stated!
Anchors
Character Classes
POSIX
Assertions
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Pcre Expression
Quantifiers
Escape Sequences
“Escaping” is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters. Common Metacharacters
Special Characters
Case Conversionl Make next character lowercase | |||||||||||||||||||||||||
PCRE regex quick reference
| [abx-z] | One character of: a, b, or the range x-z |
| [^abx-z] | One character except: a, b, or the range x-z |
| a|b | a or b |
| a? | Zero or one a’s (greedy) |
| a?? | Zero or one a’s (lazy) |
| a* | Zero or more a’s (greedy) |
| a*? | Zero or more a’s (lazy) |
| a+ | One or more a’s (greedy) |
| a+? | One or more a’s (lazy) |
| a{4} | Exactly 4 a’s |
| a{4,8} | Between (inclusive) 4 and 8 a’s |
| a{9,} | 9 or more a’s |
| (?>…) | An atomic group |
| (?=…) | A positive lookahead |
| (?!…) | A negative lookahead |
| (?<=…) | A positive lookbehind |
| (?<!…) | A negative lookbehind |
| (?:…) | A non-capturing group |
| (…) | A capturing group |
| (?P<n>…) | A capturing group named n |
Pcre Regex Cheat Sheet 2018
| ^ | Beginning of the string |
| $ | End of the string |
| d | A digit (same as [0-9]) |
| D | A non-digit (same as [^0-9]) |
| w | A word character (same as [_a-zA-Z0-9]) |
| W | A non-word character (same as [^_a-zA-Z0-9]) |
| s | A whitespace character |
| S | A non-whitespace character |
| b | A word boundary |
| B | A non-word boundary |
| n | A newline |
| t | A tab |
| cY | The control character with the hex code Y |
| xYY | The character with the hex code YY |
| uYYYY | The character with the hex code YYYY |
| . | Any character |
| Y | The Y’th captured group |
| (?1) | Recurse into numbered group 1 |
| (?&x) | Recurse into named group x |
| (?P=n) | The captured group named ‘n’ |
| (?#…) | A comment |