A regular expression is a pattern that describes a set of strings.
Regular expressions are constructed analogously to arithmetic expres-
sions, 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 using either of the first two syntaxes. In
other implementations, basic regular expressions are less powerful.
The following description applies to extended regular expressions; dif-
ferences for basic regular expressions are summarized afterwards. Perl
regular expressions add additional functionality, but the implementa-
tion used here is undocumented and is not compatible with other grep
implementations.
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 metacharacter with
special meaning may be quoted by preceding it with a backslash.
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 charac-
ters 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 dictio-
nary 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 [0-9A-Za-z], except the latter form
depends upon the C locale and the ASCII character encoding, whereas the
former is independent of locale and character set. (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 list.)
Most metacharacters lose their special meaning inside lists. 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.
The period . matches any single character. The symbol \w is a synonym
for [[:alnum:]] and \W is a synonym for [^[:alnum]].
The caret ^ and the dollar sign $ are metacharacters that respectively
match the empty string at the beginning and end of a line. 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.
A regular expression may be followed by one of several repetition oper-
ators:
? 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.
{n,m} The preceding item is matched at least n times, but not more
than m times.
Two regular expressions may be concatenated; the resulting regular
expression matches any string formed by concatenating two substrings
that respectively match the concatenated subexpressions.
Two regular expressions may be joined by the infix operator |; the
resulting regular expression matches any string matching either subex-
pression.
Repetition takes precedence over concatenation, which in turn takes
precedence over alternation. A whole subexpression may be enclosed in
parentheses to override these precedence rules.
The backreference \n, where n is a single digit, matches the substring
previously matched by the nth parenthesized subexpression of the regu-
lar expression.
In basic regular expressions the metacharacters ?, +, {, |, (, and )
lose their special meaning; instead use the backslashed versions \?,
\+, \{, \|, \(, and \).
Traditional egrep did not support the { metacharacter, and some egrep
implementations support \{ instead, so portable scripts should avoid {
in egrep patterns and should use [{] to match a literal {.
GNU egrep attempts to support traditional usage by assuming that { is
not special if it would be the start of an invalid interval specifica-
tion. For example, the shell command egrep '{1' searches for the two-
character string {1 instead of reporting a syntax error in the regular
expression. POSIX.2 allows this behavior as an extension, but portable
scripts should avoid it.
