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REGULAR EXPRESSION SEARCH

Table of Contents

Character Classes
Line terminators
Groups and capturing
Comparison to Perl 5

Brief Background

A regular expression consists of a character string where some characters are given special meaning with regard to pattern matching. Regular expressions have been in use from the early days of computing, and provide a powerful and efficient way to parse, interpret and search and replace text within an application.

Supported Syntax

	Note
	Source: https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/util/regex/Pattern.html

Table 11. Characters

`x`	The character x
`\\`	The backslash character
`\0n`	The character with octal value 0n (0 <= n <= 7)
`\0nn`	The character with octal value 0nn (0 <= n <= 7)
`\0mnn`	The character with octal value 0mnn (0 <= m <= 3, 0 <= n <= 7)
`\xhh`	The character with hexadecimal value 0xhh
`\uhhhh`	The character with hexadecimal value 0xhhhh
`\t`	The tab character ('`\u0009`')
`\n`	The newline (line feed) character ('`\u000A`')
`\r`	The carriage-return character ('`\u000D`')
`\f`	The form-feed character ('`\u000C`')
`\a`	The alert (bell) character ('`\u0007`')
`\e`	The escape character ('`\u001B`')
`\cx`	The control character corresponding to x

Table 12. Character classes

`[abc]`	`a`, `b`, or `c` (simple class)
`[^abc]`	Any character except `a`, `b`, or `c` (negation)
`[a-zA-Z]`	`a` through `z` or `A` through `Z`, inclusive (range)
`[a-d[m-p]]`	`a` through `d`, or m through `p`: `[a-dm-p]` (union)
`[a-z&&[def]]`	`d`, `e`, or `f` (intersection)
`[a-z&&[^bc]]`	`a` through `z`, except for `b` and `c`: `[ad-z]` (subtraction)
`[a-z&&[^m-p]]`	`a` through `z`, and not `m` through `p`: `[a-lq-z]`(subtraction)

Table 13. Predefined character classes

`.`	Any character (may or may not match line terminators)
`\d`	A digit: `[0-9]`
`\D`	A non-digit: `[^0-9]`
`\s`	A whitespace character: `[ \t\n\x0B\f\r]`
`\S`	A non-whitespace character: `[^\s]`
`\w`	A word character: `[a-zA-Z_0-9]`
`\W`	A non-word character: `[^\w]`

Table 14. POSIX character classes (US-ASCII only)

`\p{Lower}`	A lower-case alphabetic character: `[a-z]`
`\p{Upper}`	An upper-case alphabetic character:`[A-Z]`
`\p{ASCII}`	All ASCII:`[\x00-\x7F]`
`\p{Alpha}`	An alphabetic character:`[\p{Lower}\p{Upper}]`
`\p{Digit}`	A decimal digit: `[0-9]`
`\p{Alnum}`	An alphanumeric character:`[\p{Alpha}\p{Digit}]`
`\p{Punct}`	Punctuation: One of !"#$%&'()*+,-./:;<=>?@[\]^_`{\|}~
`\p{Graph}`	A visible character: `[\p{Alnum}\p{Punct}]`
`\p{Print}`	A printable character: `[\p{Graph}]`
`\p{Blank}`	A space or a tab: `[ \t]`
`\p{Cntrl}`	A control character: `[\x00-\x1F\x7F]`
`\p{XDigit}`	A hexadecimal digit: `[0-9a-fA-F]`
`\p{Space}`	A whitespace character: `[ \t\n\x0B\f\r]`

Table 15. Classes for Unicode blocks and categories

`\p{InGreek}`	A character in the Greek block (simple block)
`\p{Lu}`	An uppercase letter (simple category)
`\p{Sc}`	A currency symbol
`\P{InGreek}`	Any character except one in the Greek block (negation)
`[\p{L}&&[^\p{Lu}]]`	Any letter except an uppercase letter (subtraction)

Table 16. Boundary matchers

`^`	The beginning of a line
`$`	The end of a line
`\b`	A word boundary
`\B`	A non-word boundary
`\A`	The beginning of the input
`\G`	The end of the previous match
`\Z`	The end of the input but for the final terminator, if any
`\z`	The end of the input

Table 17. Greedy quantifiers

`X?`	X, once or not at all
`X*`	X, zero or more times
`X+`	X, one or more times
`X{n}`	X, exactly n times
`X{n,}`	X, at least n times
`X{n,m}`	X, at least n but not more than m times

Table 18. Reluctant quantifiers

`X??`	X, once or not at all
`X*?`	X, zero or more times
`X+?`	X, one or more times
`X{n}?`	X, exactly n times
`X{n,}?`	X, at least n times
`X{n,m}?`	X, at least n but not more than m times

Table 19. Possessive quantifiers

`X?+`	X, once or not at all
`X*+`	X, zero or more times
`X++`	X, one or more times
`X{n}+`	X, exactly n times
`X{n,}+`	X, at least n times
`X{n,m}+`	X, at least n but not more than m times

Table 20. Logical operators

`XY`	X followed by Y
`X\|Y`	Either X or Y
`(X)`	X, as a capturing group

Table 21. Back references

`\n`	Whatever the n^th capturing group matched

Table 22. Quotation

`\`	Nothing, but quotes the following character
`\Q`	Nothing, but quotes all characters until `\E`
`\E`	Nothing, but ends quoting started by `\Q`

Table 23. Special constructs (non-capturing)

`(?:X)`	X, as a non-capturing group
`(?idmsux-idmsux)`	Nothing, but turns match flags on - off
`(?idmsux-idmsux:X)`	X, as a non-capturing group with the given flags on - off
`(?=X)`	X, via zero-width positive lookahead
`(?!X)`	X, via zero-width negative lookahead
`(?<=X)`	X, via zero-width positive lookbehind
`(?<!X)`	X, via zero-width negative lookbehind
`(?>X)`	X, as an independent, non-capturing group

Backslashes, escapes, and quoting

The backslash character ('\') serves to introduce escaped constructs, as defined in the table above, as well as to quote characters that otherwise would be interpreted as unescaped constructs. Thus the expression \\ matches a single backslash and \{ matches a left brace.

It is an error to use a backslash prior to any alphabetic character that does not denote an escaped construct; these are reserved for future extensions to the regular-expression language. A backslash may be used prior to a non-alphabetic character regardless of whether that character is part of an unescaped construct.

Backslashes within string literals in Java source code are interpreted as required by the Java Language Specification as either Unicode escapes or other character escapes. It is therefore necessary to double backslashes in string literals that represent regular expressions to protect them from interpretation by the Java byte code compiler. The string literal "\b", for example, matches a single backspace character when interpreted as a regular expression, while "\\b" matches a word boundary. The string literal "$hello$" is illegal and leads to a compile-time error; in order to match the string (hello) the string literal "\$hello\$" must be used.