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 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 the 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.

Further Reading #

Regex101 - Regular Expression tester and debugger