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NAME
grep, egrep, fgrep - print lines matching a pattern
SYNOPSIS
grep [options] PATTERN [FILE...]
grep [options] [-e PATTERN | -f FILE] [FILE...]
DESCRIPTION
Grep searches the named input FILEs (or standard input if no files are named, or
the file name - is given) for lines containing a match to the given PATTERN. By
default, grep prints the matching lines.
In addition, two variant programs egrep and fgrep are available. Egrep is the
same as grep -E. Fgrep is the same as grep -F.
.................
-E, --extended-regexp
Interpret PATTERN as an extended regular expression (see below).
REGULAR EXPRESSIONS
A regular expression is a pattern that describes a set of strings. Regular
expressions are constructed analogously to arithmetic expressions, by using vari-
ous operators to combine smaller expressions.
Grep understands two different versions of regular expression syntax: "basic" and
"extended." In GNU grep, there is no difference in available functionality using
either syntax. In other implementations, basic regular expressions are less pow-
erful. The following description applies to extended regular expressions; differ-
ences for basic regular expressions are summarized afterwards.
The fundamental building blocks are the regular expressions that match a single
character. Most characters, including all letters and digits, are regular expres-
sions that match themselves. Any metacharacter with special meaning may be quoted
by preceding it with a backslash.
A list of characters enclosed by [ and ] matches any single character in that
list; if the first character of the list is the caret ^ then it matches any char-
acter not in the list. For example, the regular expression [0123456789] matches
any single digit. A range of characters may be specified by giving the first and
last characters, separated by a hyphen. Finally, certain named classes of charac-
ters are predefined. 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 POSIX 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 metachar-
acters 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 \> respec-
tively 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 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.
{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 regu-
lar expression matches any string matching either subexpression.
Repetition takes precedence over concatenation, which in turn takes precedence
over alternation. A whole subexpression may be enclosed in parentheses to over-
ride these precedence rules.
The backreference \n, where n is a single digit, matches the substring previously
matched by the nth parenthesized subexpression of the regular 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 implementa-
tions 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 specification. 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.
Grep problem
