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Learn X in Y minutes

Where X=PCRE

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

  \      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+

All these examples can be tried at https://regex101.com/

  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

Further Reading


Got a suggestion? A correction, perhaps? Open an Issue on the Github Repo, or make a pull request yourself!

Originally contributed by Sachin Divekar, and updated by 0 contributor(s).