I am using regular expressions in PHP scripts and need to look at reference document frequently.

If you are writing PHP scripts and using regular expressions, you need to bookmark this PHP official reference document: "Regular Expressions (Perl-Compatible)".

Topics covered in "Regular Expressions (Perl-Compatible)" include:

Introduction
Installing/Configuring
  Requirements
  Installation
  Runtime Configuration
  Resource Types
Predefined Constants
Examples
PCRE Patterns
  PCRE regex syntax — Pattern Syntax
  Possible modifiers in regex patterns — Pattern Modifiers
  Differences From Perl — Perl Differences
  Differences from POSIX regex
PCRE Functions
  preg_filter — Perform a regular expression search and replace
  preg_grep — Return array entries that match the pattern
  preg_last_error — Returns the error code of the last PCRE regex execution
  preg_match_all — Perform a global regular expression match
  preg_match — Perform a regular expression match
  preg_quote — Quote regular expression characters
  preg_replace_callback — Perform a regular expression search and replace using a callback
  preg_replace — Perform a regular expression search and replace
  preg_split — Split string by a regular expression