--- 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