Regular Expressions in PHP

Supported RE’s The most important set of regex functions start with preg. These functions are a PHP wrapper around the PCRE library (Perl- Compatible Regular Expressions)PCRE library (Perl- Compatible Regular Expressions The oldest set of regex functions are those that start with ereg. They implement POSIX Extended Regular Expressions. Portable Operating System Interface [for Unix] POSIX Extended Regular Expressions

PCRE library (Perl-Compatible Regular Expressions When using the PCRE functions, it is required that the pattern is enclosed by delimiters. A delimiter can be any non-alphanumeric, non- backslash, non-whitespace character. Often used delimiters are forward slashes (/), hash signs (#) and tildes (~). The following are all examples of valid delimited patterns. –/foo bar/ –#^[^0-9]$# –+php+ –%[a-zA-Z0-9_-]%

Delimiters If the delimiter needs to be matched inside the pattern it must be escaped using a backslash. If the delimiter appears often inside the pattern, it is a good idea to choose another delimiter in order to increase readability. –/ – #

Meta-characters The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of meta- characters. Some of the meta chars are.. –\ general escape character with several uses –^ assert start of subject (or line, in multiline mode) –$ assert end of subject (or line, in multiline mode) –. match any character except newline (by default) –[ start character class Definition

Meta Chars contd.. ] –end character class definition | –start of alternative branch ( –start subpattern ) –end subpattern ? –extends the meaning of (, also 0 or 1 quantifier, also makes greedy quantifiers lazy * –0 or more quantifier + –1 or more quantifier { –start min/max quantifier

} –end min/max quantifier Part of a pattern that is in square brackets is called a "character class". In a character class the only meta- characters are: \ –general escape character ^ –negate the class, but only if the first character - –indicates character range ] –terminates the character class

Escape Sequences The backslash character has several uses. Firstly, if it is followed by a non- alphanumeric character, it takes away any special meaning that character may have. –This use of backslash as an escape character applies both inside and outside character classes. –For example, if you want to match a "*" character, you write "\*" in the pattern.

A second use of backslash provides a way of encoding non-printing characters in patterns in a visible manner \a –alarm, that is, the BEL character (hex 07) \f –formfeed (hex 0C) \n –newline (hex 0A) \t –tab (hex 09)

PCRE Functions preg_filter — Perform a regular expression search and replacepreg_filter preg_grep — Return array entries that match the patternpreg_grep preg_last_error — Returns the error code of the last PCRE regex executionpreg_last_error preg_match_all — Perform a global regular expression matchpreg_match_all preg_match — Perform a regular expression matchpreg_match preg_quote — Quote regular expression characterspreg_quote preg_replace_callback — Perform a regular expression search and replace using a callbackpreg_replace_callback preg_replace — Perform a regular expression search and replacepreg_replace preg_split — Split string by a regular expressionpreg_split

POSIX Regex Functions ereg_replace — Replace regular expressionereg_replace ereg — Regular expression matchereg eregi_replace — Replace regular expression case insensitiveeregi_replace eregi — Case insensitive regular expression matcheregi split — Split string into array by regular expressionsplit spliti — Split string into array by regular expression case insensitivespliti sql_regcase — Make regular expression for case insensitive matchsql_regcase

ereg (PHP 4, PHP 5) ereg — Regular expression match Syntax –int ereg ( string $pattern, string $string [, array &$regs ] ) Searches a string for matches to the regular expression given in pattern in a case-sensitive way. –pattern Case sensitive regular expression. –string The input string. –regs If matches are found for parenthesized substrings of pattern and the function is called with the third argument regs, the matches will be stored in the elements of the array regs. $regs[1] will contain the substring which starts at the first left parenthesis; $regs[2] will contain the substring starting at the second, and so on. $regs[0] will contain a copy of the complete string matched.

Example code snippet takes a date in ISO format (YYYY- MM-DD) and prints it in DD.MM.YYYY format:

eregi (PHP 4, PHP 5) eregi — Case insensitive regular expression match Syntax –int eregi ( string $pattern, string $string [, array &$regs ] ) This function is identical to ereg() except that it ignores case distinction when matching alphabetic characters.ereg() Example –

Differences from POSIX regex As of PHP 5.3.0, the POSIX Regex extension is deprecated.POSIX Regex There are a number of differences between POSIX regex and PCRE regex –The PCRE functions require that the pattern is enclosed by delimiters.delimiters –Unlike POSIX, the PCRE extension does not have dedicated functions for case-insensitive matching. Instead, this is supported using the /i pattern modifierpattern modifier –The POSIX functions find the longest of the leftmost match, but PCRE stops on the first valid match.

Function Replacements

References PCRE Functions – POSIX Regex Functions – String Functions –