Converse Reference and sub-mode analysis of PHP regular expressions _ regular expressions

One of the most important features of regular expressions is the ability to store portions of a successful pattern for later use.
adding parentheses () to either side of a regular expression pattern or partial pattern can store this part of the expression in a temporary buffer.

Each captured child match is stored sequentially according to what is encountered from left to right in the regular expression pattern.
The buffer number for the storage child match starts at 1 and is numbered consecutively to the maximum of 99 subexpression.
Each buffer can be accessed using ' \ n ' (or ' $n '), where n is an Arabic numeral of 1 to 99, used to identify a specific buffer (subexpression) sequentially.

Example 1: The simplest and most useful example is the determination of the position of two consecutive words in the text

Copy Code code as follows:

<?php
$string = "gasoline going Up";
$pattern = "/\b ([a-z]+) \\1\b/i"; The \\1 here cannot use \$1 or $
$str = Preg_replace ($pattern, "\\1", $string); Here the \\1 can use \$1 or $ to refer to the first child match
Echo $str; Effect is the ' cost ' of gasoline going up
?>

The subexpression in the example is the item in parentheses. \b matches the start or end of a word. + Match repeat one or more times.
The subexpression matches one or more alphabetic characters, which are matched by ' [a-z]+ '.

The second part of the regular expression is a reference to the previously captured substring, that is, the second occurrence of a word that is matched by an additional expression, using ' \\1 ' to refer to the first child match, and the first \ is an escape character.

I is the modifier in the regular expression. I: Ignore case.

Example 2:

The reverse reference ($0-99 or \\0-99) and child mode of regular expressions begin with (/()/).
Here, $ is a match for all matching patterns. Is the 1th child match, and the $ to $99 sequence is the 2nd to 99th Child match.
When you match a $1-99 to a reference, if the following character is a number, separate it with curly braces. Example: ${1}1.

function
Mixed preg_replace (mixed pattern, mixed replacement, mixed subject [, int limit])

function
Searches the subject for a match in pattern mode and replaces it with replacement. If limit is specified, only the limit match is substituted, and if limit is omitted or the value is-1, all occurrences are replaced.
Replacement can contain a \\n form or a reverse reference in $n form, where n can represent 0 to 99,\\n text that matches pattern nth, and \\0 represents text that matches the entire patterns.

Child Mode
The regular expression enclosed in parentheses in the $pattern argument, the number of child modes, which is the number of parentheses from left to right. (Pattern is mode)

Example

Copy Code code as follows:

<?php
$time =date ("y-m-d h:i:s");
$pattern = "/(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})/I";
$replacement = "\ $time format for:$0<br> is replaced by $ $ $ $ $ $ $ $ per year;
Print Preg_replace ($pattern, $replacement, $time);
?>

Output:
$time format: 2011-07-25 17:59:26
The replacement format is: July 25, 2011 17:59 26 seconds

Accompanying regular expression sign table

character	Description
\	Marks the next character as a special character, or a literal character, or a backward reference, or a octal escape character. For example, ' n ' matches the character ' n '. ' \ n ' matches a newline character. Sequence ' \ ' matches ' \ ' and ' \ (' Matches ' (".
^	Matches the start position of the input string. If the Multiline property of the RegExp object is set, ^ also matches the position after ' \ n ' or ' \ R '.
$	Matches the end position of the input string. If the Multiline property of the RegExp object is set, the $ also matches the position before ' \ n ' or ' \ R '.
*	Matches the preceding subexpression 0 or more times. For example, zo* can match "z" and "Zoo". * is equivalent to {0,}.
+	Matches the preceding subexpression one or more times. For example, ' zo+ ' can match "Zo" and "Zoo", but cannot match "Z". + is equivalent to {1,}.
?	Match the preceding subexpression 0 times or once. For example, "Do (es)" can match "do" in "do" or "does". is equivalent to {0,1}.
N	n is a non-negative integer. Matches the determined n times. For example, ' o{2} ' cannot match ' o ' in ' Bob ', but can match two o in ' food '.
{N,}	n is a non-negative integer. Match at least n times. For example, ' o{2,} ' cannot match ' o ' in ' Bob ' but can match all o in ' Foooood '. ' O{1,} ' is equivalent to ' o+ '. ' O{0,} ' is equivalent to ' o* '.
{N,m}	M and n are nonnegative integers, of which n <= M. Matches n times at least and matches up to M times. For example, "o{1,3}" will match the first three o in "Fooooood". ' o{0,1} ' is equivalent to ' o '. Notice that there is no space between the comma and the two number.
?	When the character is immediately following any of the other qualifiers (*, +,?, {n}, {n,}, {n,m}), the matching pattern is not greedy. Non-greedy patterns match as few strings as possible, while the default greedy pattern matches as many of the searched strings as possible. For example, for the string "oooo", ' o+? ' will match a single "O", and ' o+ ' will match all ' o '.
.	Matches any single character except "\ n". To match any character including ' \ n ', use a pattern like ' [. \ n] '.
(pattern)	Match pattern and get this match. The obtained matches can be obtained from the resulting matches collection, use the Submatches collection in VBScript, and use the $0...$9 property in JScript. To match the parentheses character, use ' \ (' or ' \ ').
(?:p Attern)	Matches pattern but does not get a matching result, which means it is a non fetch match and is not stored for later use. This is useful for combining parts of a pattern with the "or" character (|). For example, ' Industr (?: y|ies) is a more abbreviated expression than ' industry|industries '.
(? =pattern)	Forward lookup, matching the find string at the beginning of any string matching pattern. This is a non-fetch match, that is, the match does not need to be acquired for later use. For example, ' Windows (? =95|98| nt|2000) ' Can match windows in Windows 2000, but cannot match windows in Windows 3.1. It does not consume characters, that is, after a match occurs, the next matching search begins immediately after the last match, instead of starting after the character that contains the pre-check.
(?! Pattern	Negative pre-check, matches the lookup string at the beginning of any mismatched pattern string. This is a non-fetch match, that is, the match does not need to be acquired for later use. For example, ' Windows (?! 95|98| nt|2000) ' Can match windows in Windows 3.1, but cannot match windows in Windows 2000. It does not consume characters, that is, after a match occurs, the next matching search begins immediately after the last match, instead of starting after the character that contains the pre-check.
X|y	Match x or Y. For example, ' Z|food ' can match "z" or "food". ' (z|f) Ood ' matches ' zood ' or ' food '.
[XYZ]	Character set combination. Matches any one of the characters contained. For example, ' [ABC] ' can match ' a ' in ' plain '.
[^XYZ]	Negative character set combination. Matches any characters that are not included. For example, ' [^ABC] ' can match ' P ' in ' plain '.
[A-z]	The range of characters. Matches any character within the specified range. For example, ' [A-z] ' can match any lowercase alphabetic character in the range ' a ' to ' Z '.
[^a-z]	Negative character range. Matches any character that is not in the specified range. For example, ' [^a-z] ' can match any character that is not in the range of ' a ' to ' Z '.
\b	Matches a word boundary, which is the position between the word and the space. For example, ' er\b ' can match ' er ' in ' never ', but cannot match ' er ' in ' verb '.
\b	Matches a non-word boundary. ' er\b ' can match ' er ' in ' verb ', but cannot match ' er ' in ' Never '.
\cx	Matches the control character indicated by X. For example, \cm matches a control-m or carriage return character. The value of x must be one-a-Z or a-Z. Otherwise, c is treated as a literal ' C ' character.
\d	Matches a numeric character. equivalent to [0-9].
\d	Matches a non-numeric character. equivalent to [^0-9].
\f	Matches a page feed character. Equivalent to \x0c and \CL.
\ n	Matches a line feed character. Equivalent to \x0a and \CJ.
\ r	Matches a carriage return character. Equivalent to \x0d and \cm.
\s	Matches any white space character, including spaces, tabs, page breaks, and so on. equivalent to [\f\n\r\t\v].
\s	Matches any non-white-space character. equivalent to [^ \f\n\r\t\v].
\ t	Matches a tab character. Equivalent to \x09 and \ci.
\v	Matches a vertical tab. Equivalent to \x0b and \ck.
\w	Matches any word character that includes an underscore. Equivalent to ' [a-za-z0-9_] '.
\w	Matches any non word character. Equivalent to ' [^a-za-z0-9_] '.
\xn	Matches n, where n is the hexadecimal escape value. The hexadecimal escape value must be a determined two digits long. For example, ' \x41 ' matches ' A '. ' \x041 ' is equivalent to ' \x04 ' & ' 1 '. You can use ASCII encoding in regular expressions ...
\num	Matches num, where num is a positive integer. A reference to the match that was obtained. For example, ' (.) \1 ' matches two consecutive identical characters.
\ n	Identifies a octal escape value or a backward reference. n is a backward reference if you have at least n obtained subexpression before \ nthe. Otherwise, if n is an octal number (0-7), then N is an octal escape value.
\nm	Identifies a octal escape value or a backward reference. NM is a backward reference if at least NM has obtained the subexpression before \nm. If there are at least N fetches before \nm, then n is a backward reference followed by a literal m. If all the preceding conditions are not satisfied, if both N and M are octal digits (0-7), then \nm will match octal escape value nm.
\nml	If n is an octal number (0-3) and both M and L are octal digits (0-7), the octal escape value NML is matched.
\un	matches N, where N is a Unicode character represented in four hexadecimal digits. For example, \u00a9 matches the copyright symbol (?).