grep

search input files for lines that match a given pattern.

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.

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 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 expres-
sions are less powerful. The following description applies to extended regular expressions; differences 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 expressions 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 character 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 charac-
ters, separated by a hyphen. Finally, certain named classes of characters are predefined:

       [:alnum:]  - Any digit or Alphanumeric
       [:alpha:]  - Any Alphanumeric
       [:cntrl:]  - octal codes 000 through 037, or `DEL' (octal 177)
       [:digit:]  - Any one of `0 1 2 3...7 8 9'
       [:graph:]  - Anything that is not a `[:alnum:]' or `[:punct:]'
       [:lower:]  - Any one of `a b c... x y z'
       [:print:]  - Any char from the `[:space:]' class, and any char not in the `[:graph:]' class.
       [:punct:]  - Any one of `! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~'
       [:space:]  - Any one of `CR FF HT NL VT SPACE'
       [:upper:]  - Any one of `A B C... X Y Z'
       [:xdigit:] - Hex:  `a b c d e f A B C D E F 0 1 2 3 4 5 6 7 8 9'

       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 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 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 concate-
nating 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 subexpression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subex-
pression 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 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 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  assum-
       ing  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.

"Talent is hitting a target no-one else can hit, genius is hitting a target no-one else can see" ~ Schopenhauer

Related:

awk - Find and Replace text within file(s)
grep - Search file(s) for lines that match a given pattern
tr - Translate, squeeze, and/or delete characters


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