POSIX Basic Regular Expression!s

- 원문 : http://donkey612.blog.me/70078012834

POSIX Basic Regular Expression!s

Traditional Unix regular expression! syntax followed common conventions

but often differed from tool to tool. The IEEE POSIX Basic Regular Expression!s (BRE)

standard (released alongside an alternative flavor called Extended Regular Expression!s or ERE)

was designed mostly for backward compatibility with the traditional (Simple Regular Expression!)

syntax but provided a common standard which has since been adopted as the default syntax

of many Unix regular expression! tools, though there is often some variation or additional features.

Many such tools also provide support for ERE syntax with command line arguments.

 

In the BRE syntax, most characters are treated as literals — they match only themselves

(i.e., a matches "a"). The exceptions, listed below, are called metacharacters or metasequences.

 

.	Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor, character encoding, and platform specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expression!s, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
[ ]	A bracket expression!. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", and "z", as does [a-cx-z]. The - character is treated as a literal character if it is the last or the first character within the brackets, or if it is escaped with a backslash: [abc-], [-abc], or [a\-bc].
[^ ]	Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". As above, literal characters and ranges can be mixed.
^	Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
$	Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
\( \)	Defines a marked subexpression!. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression! is also called a block or capturing group.
\n	Matches what the nth marked subexpression! matched, where n is a digit from 1 to 9. This construct is theoretically irregular and was not adopted in the POSIX ERE syntax. Some tools allow referencing more than nine capturing groups.
*	Matches the preceding element zero or more times. For example, ab*c matches "ac", "abc", "abbbc", etc. [xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on. \(ab\)* matches "", "ab", "abab", "ababab", and so on.
\{m,n\}	Matches the preceding element at least m and not more than n times. For example, a\{3,5\} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few, older instances of regular expression!s.

 

Examples:

.at matches any three-character string ending with "at", including "hat", "cat", and "bat".

[hc]at matches "hat" and "cat".

[^b]at matches all strings matched by .at except "bat".

^[hc]at matches "hat" and "cat", but only at the beginning of the string or line.

[hc]at$ matches "hat" and "cat", but only at the end of the string or line.

 

POSIX Extended Regular Expression!s

 

The meaning of metacharacters escaped with a backslash is reversed for some characters

in the POSIX Extended Regular Expression! (ERE) syntax. With this syntax, a backslash causes

the metacharacter to be treated as a literal character. Additionally, support is removed for

\n backreferences[citation needed] and the following metacharacters are added:

 

?	Matches the preceding element zero or one time. For example, ba? matches "b" or "ba".
+	Matches the preceding element one or more times. For example, ba+ matches "ba", "baa", "baaa", and so on.
|	The choice (aka alternation or set union) operator matches either the expression! before or the expression! after the operator. For example, abc|def matches "abc" or "def".

 

Examples:

[hc]+at matches "hat", "cat", "hhat", "chat", "hcat", "ccchat", and so on, but not "at".

[hc]?at matches "hat", "cat", and "at".

cat|dog matches "cat" or "dog".

POSIX Extended Regular Expression!s can often be used with modern Unix utilities by including thecommand line flag -E.

 

POSIX character classes

 

Since many ranges of characters depend on the chosen locale setting

(i.e., in some settings letters are organized as abc...zABC...Z, while in some others as aAbBcC...zZ),

the POSIX standard defines some classes or categories of characters as shown in the following

table:

POSIX	Perl	ASCII	Description
[:alnum:]		[A-Za-z0-9]	Alphanumeric characters
[:word:]	\w	[A-Za-z0-9_]	Alphanumeric characters plus "_"
	\W	[^\w]	non-word character
[:alpha:]		[A-Za-z]	Alphabetic characters
[:blank:]		[ \t]	Space and tab
[:cntrl:]		[\x00-\x1F\x7F]	Control characters
[:digit:]	\d	[0-9]	Digits
	\D	[^\d]	non-digit
[:graph:]		[\x21-\x7E]	Visible characters
[:lower:]		[a-z]	Lowercase letters
[:print:]		[\x20-\x7E]	Visible characters and spaces
[:punct:]		[-!"#$%&'()*+,./:;<=>?@[\\\]_`{|}~]	Punctuation characters
[:space:]	\s	[ \t\r\n\v\f]	Whitespace characters
	\S	[^\s]	non-whitespace character
[:upper:]		[A-Z]	Uppercase letters
[:xdigit:]		[A-Fa-f0-9]	Hexadecimal digits

 

POSIX character classes can only be used within bracket expression!s.

For example, [[:upper:]ab] matches the uppercase letters and lowercase "a" and "b".

 

In Perl regular expression!s, [:print:] matches [:graph:] union [:space:].

An additional non-POSIX class understood by some tools is [:word:], which is usually defined as

[:alnum:] plus underscore. This reflects the fact that in many programming languages these are

the characters that may be used in identifiers. The editor Vim further distinguishes word

and word-head classes (using the notation \w and \h) since in many programming languages

the characters that can begin an identifier are not the same as those that can occur

in other positions.

Note that what the POSIX regular expression! standards call character classes are commonly referred to as POSIX character classes in other regular expression! flavors which support them. With most other regular expression! flavors, the term character class is used to describe what POSIX calls bracket expression!s.