正则表达式简介

^

Matches the starting position within the string. In line-based tools, it

matches the starting position of any line.

.

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 expressions, 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", or "z", as does [a-cx-z]. The - character is treated as a literal character if it is the last or the first (after the ^, if present) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].

[^ ]

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". Likewise, literal characters and ranges can be mixed.

$

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. BRE¹ mode requires \( \).

\n

Matches what the nth marked subexpression matched, where n is a digit

from 1 to 9. This construct is vaguely defined in the POSIX.2 standard. 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 regexes. BRE mode requires \{m,n\}.