正则表达式
REGULAR EXPRESSIONS
A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions.
grep understands three different versions of regular expression syntax: “basic,” “extended” and “perl.” In GNU grep, there is no difference in available
functionality between basic and extended syntaxes. In other implementations, basic regular expressions are less powerful. The following description applies to
extended regular expressions; differences for basic regular expressions are summarized afterwards. Perl regular expressions give additional functionality, and are
documented in pcresyntax(3) and pcrepattern(3), but may not be available on every system.
The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular
expressions that match themselves. Any meta-character with special meaning may be quoted by preceding it with a backslash.
The period . matches any single character.
Character Classes and Bracket Expressions
A bracket expression is a list of characters enclosed by [ and ]. It matches any single character in that list; if the first character of the list is the caret ^
then it matches any character not in the list. For example, the regular expression [0123456789] matches any single digit.
Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two
characters, inclusive, using the locale’s collating sequence and character set. For example, in the default C locale, [a-d] is equivalent to [abcd]. Many locales
sort characters in dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be equivalent to [aBbCcDd], for example. To obtain
the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value C.
Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their names are self explanatory, and they are [:alnum:],
[:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. For example, [[:alnum:]] means the character
class of numbers and letters in the current locale. In the C locale and ASCII character set encoding, this is the same as [0-9A-Za-z]. (Note that the brackets in
these class names are part of the symbolic names, and must be included in addition to the brackets delimiting the bracket expression.) Most meta-characters lose
their special meaning inside bracket expressions. To include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but
first. Finally, to include a literal - place it last.
Anchoring
The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line.
The Backslash Character and Special Expressions
The symbols < and > respectively match the empty string at the beginning and end of a word. The symbol \b matches the empty string at the edge of a word, and \B
matches the empty string provided it’s not at the edge of a word. The symbol \w is a synonym for [[:alnum:]] and \W is a synonym for [^[:alnum:]].
Repetition
A regular expression may be followed by one of several repetition operators:
? The preceding item is optional and matched at most once.
* The preceding item will be matched zero or more times.
+ The preceding item will be matched one or more times.
{n} The preceding item is matched exactly n times.
{n,} The preceding item is matched n or more times.
{,m} The preceding item is matched at most m times. This is a GNU extension.
{n,m} The preceding item is matched at least n times, but not more than m times.
Concatenation
Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the
concatenated expressions.
Alternation
Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either alternate expression.
Precedence
Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole expression may be enclosed in parentheses to override
these precedence rules and form a subexpression.
Back References and Subexpressions
The back-reference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression.
Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions ?, +, {, |, (, and ).
Traditional egrep did not support the { meta-character, and some egrep implementations support { instead, so portable scripts should avoid { in grep -E patterns
and should use [{] to match a literal {.
GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification. For example, the
command grep -E ’{1’ searches for the two-character string {1 instead of reporting a syntax error in the regular expression. POSIX allows this behavior as an
extension, but portable scripts should avoid it.