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---
language: PCRE
filename: pcre.txt
contributors:
- ["Sachin Divekar", "http://github.com/ssd532"]
---
A regular expression (regex or regexp for short) is a special text string for describing a search pattern. e.g. to extract domain name from a string we can say `/^[a-z]+:/` and it will match `http:` from `http://github.com/`.
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 including PHP, Apache, R to name a few.
There are two different sets of metacharacters:
* Those that are recognized anywhere in the pattern except within square brackets
```
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
```
* Those that are recognized within square brackets. Outside square brackets. They are also called as character classes.
```
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX syntax)
] terminates the character class
```
PCRE provides some generic character types, also called as character classes.
```
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space character
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
\V any character that is not a vertical white space character
\w any "word" character
\W any "non-word" character
```
## Examples
We will test our examples on following string `66.249.64.13 - - [18/Sep/2004:11:07:48 +1000] "GET /robots.txt HTTP/1.0" 200 468 "-" "Googlebot/2.1"`. It is a standard Apache access log.
| Regex | Result | Comment |
| :---- | :-------------- | :------ |
| `GET` | GET | GET matches the characters GET literally (case sensitive) |
| `\d+.\d+.\d+.\d+` | 66.249.64.13 | `\d+` match a digit [0-9] one or more times defined by `+` quantifier, `\.` matches `.` literally |
| `(\d+\.){3}\d+` | 66.249.64.13 | `(\d+\.){3}` is trying to match group (`\d+\.`) exactly three times. |
| `\[.+\]` | [18/Sep/2004:11:07:48 +1000] | `.+` matches any character (except newline), `.` is any character |
| `^\S+` | 66.249.64.13 | `^` means start of the line, `\S+` matches any number of non-space characters |
| `\+[0-9]+` | +1000 | `\+` matches the character `+` literally. `[0-9]` character class means single number. Same can be achieved using `\+\d+` |
## Further Reading
[Regex101](https://regex101.com/) - Regular Expression tester and debugger
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