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Diffstat (limited to 'pcre.html.markdown')
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1 files changed, 24 insertions, 23 deletions
diff --git a/pcre.html.markdown b/pcre.html.markdown index 0b61653d..9e091721 100644 --- a/pcre.html.markdown +++ b/pcre.html.markdown @@ -3,16 +3,18 @@ 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/`. +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) @@ -32,18 +34,19 @@ There are two different sets of metacharacters: ``` * 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. +``` + +PCRE provides some generic character types, also called as character classes. + ``` \d any decimal digit \D any character that is not a decimal digit @@ -59,24 +62,22 @@ PCRE provides some generic character types, also called as character classes. ## 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. +We will test our examples on the following string: -| 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+` | - -All these examples can be tried at https://regex101.com/ +``` +66.249.64.13 - - [18/Sep/2004:11:07:48 +1000] "GET /robots.txt HTTP/1.0" 200 468 "-" "Googlebot/2.1" +``` -1. Copy the example string in `TEST STRING` section -2. Copy regex code in `Regular Expression` section -3. The web application will show the matching result + 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 |