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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 the protocol from a url string we can say /^[a-z]+:/ and it will match http: from

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:

  \      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


We will test our examples on the following string: - - [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+ \d+ match a digit [0-9] one or more times defined by + quantifier, \. matches . literally
(\d+\.){3}\d+ (\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+ ^ 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 - Regular Expression tester and debugger

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 3 contributors.