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