Regular expression syntax and precedence for regular expressions tutorial

Regular expression syntax

A regular expression is a literal pattern consisting of ordinary characters (such as characters A through Z) and special characters (called metacharacters ). This pattern describes one or more strings to be matched when looking for a text body. A regular expression is used as a template to match a character pattern with the string being searched for.

Here are some examples of regular expressions that you might encounter:

JScript	VBScript	Match
/^\[\t]*$/	"^\[\t]*$"	Matches a blank row.
/\d{2}-\d{5}/	"\d{2}-\d{5}"	Verify that an ID number consists of a 2-digit number, a hyphen, and a 5-digit number.
/< (. *) >.*<\/\1>/	"< (. *) >.*<\/\1>"	Matches an HTML tag.

The following table is a complete list of metacharacters and its behavior in the context of regular expressions:

character	Description
\	Marks the next character as a special character, or a literal character, or a back 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. Liu, "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 other qualifier (*, +,?, {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 ' \ ').
(?:pattern)	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, at the beginning of any mismatch negative lookahead matches the search string at either point where a string does matching pattern With a lookup 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 back reference. N is a back-reference if at least N obtained subexpression before \n . Otherwise, if n is an octal number (0-7), then N is an octal escape value.
\nm	Identifies a octal escape value or a back reference. If \nm has at least one preceded by at least nm , then nm is a back-reference. If at least N is fetched before \nm , then N is a back reference followed by a literal m . If the preceding conditions are not satisfied, if both n and m are octal digits (0-7), thennm will match the 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 (?).

Priority order of regular expressions

After a regular expression is constructed, it can be evaluated like a mathematical expression, that is, it can be evaluated from left to right and in a priority order.

The following table lists the order of precedence for various regular expression operators from highest priority to lowest priority:

operator	Description
\	Escape character
(), (?:), (?=), []	Parentheses and square brackets
*, +,?, {n}, {n,}, {n,m}	Qualifier
^, $, \Anymetacharacter	Location and order
|	"or" action