grep

The matching substring must either be at the beginning of the line, or preceded by a non-alphanumeric character and
must be either at the end of the line or followed by a non-alphanumeric character.

Alphanumeric characters are letters, digits, and the underscore.

The matched sections can be displayed in color as per $GREP_COLOR in the form ff[;bb]
where ff is the code used for the foreground and bb is used for the background.

Examples:
magenta on yellow 35;43 blue on cyan 34;46 red 31 (default)
Foreground: black 30, green 32, yellow 33, blue 34, magenta 35, cyan 36, white 37, red 31 (default)
Background: black 40, green 42, yellow 43, blue 44, magenta 45, underscore 4, [blink] 5, inverse 7
export GREP_COLOR="33;44"

Codes are not output when inappropriate (For example: to a file or pipe) unless when=always

grep --color=always birds logfile | more -r

 grep -c images *html |grep -v 0$ #dont show lines with 0 count
BackupStrageties.html:1
bash-usingHistory.html:1
Characters.html:2
clonezilla.1.html:6
colorNames.html:1
colors.html:3
css-box.html:4

Prefix each line with the line number

Used with commands find -print0, perl -0, sort -z, and xargs -0.

gzip -cd foo.gz |grep --label=foo something

Example:

> while grep -B 2 -m 1 bod; do echo ; done Outputs any trailing context lines.
With -c or --count , does not output a count greater than num.

With -v or --invert-match , stops after outputting num non-matching lines.

type
binary and
without-match assume binary file does not match, default.
text processe all files as text; equivalent to -a .

Control characters sent to the terminal can set attributes making it unreadable. If this happens try STTY SANE

If the file appears to be a text file, ␍ s are ignored to make regular expressions with ^ and $ work correctly.
Causes all files to be read and passed to the matching mechanism verbatim;
if the file is a text file with ␍␊ pairs at the end of each line, this will cause some regular expressions to fail. only under MS-Windows unless used with -b .

By default, under MS-Windows, uses the first 32KB of the file to determine if a file is text.

International Components for Unicode

Grep understands two different versions of regular expression syntax: basic and extended.
GNU grep, there is no difference.

The simplest regular expression matchs a single character.  For example: a matches an a , R matches an R
Regular expresions can be combined.  For example: AR matches AR

Regular expressions joined by | match any string matching either expression.

Metacharacters
These can be treated as normal character by preceding it with a \  (backslash) (the opposite of BRE)!

.  (period) matches any single character.
^  (caret) matches the beginning of a line
$  (dollar sign) matches the end of a line.

repetition operators may follow a regular expression.

/9\{2,3\} bytes/ matches 99 bytes and 999 bytes

• bracket expressions match a single character.
  
  • list of characters enclosed by [ and ] matches any single character in the list
    Most metacharacters have no special meaning inside lists.
     To include ] place it first in the list. ][)(
     To include ^ place it anywhere but first. Otherwise it matches the beginning of the line.
     To include - place it last, lest it be considered a range
    For example: [12345689] matches any single digit except 0 or 7.
    
  • nonmatching list
    ^  (caret) preceeding the expression matchs any character not in the list.
    For example:
    ^[abcdefghijklmnopqrstuvwxyz] matches a single non-lowercase character.
  • range expression
    - (hyphen) seperating two characters matches any single character that sorts between the two characters, inclusive.
    using the locale's collating sequence and character set.
    For example:
    [3-7] is equivalent to [34567].
    Many locales sort characters in dictionary order, in these locales [a-d] is typically equivalent [aBbCcDd] not [abcd].
    To obtain the traditional interpretation of bracket expressions ( where [a-d] is equivalent to [abcd]), use the C locale using set LC_ALL=C.
• named classes match a single character
  [:alnum:]   [:alpha:]   [:cntrl:]   [:digit:]   [:graph:]   [:lower:]   [:print:]   [:punct:]   [:space:]   [:upper:]   and [:xdigit:]
      Brackets in these class names are part of the symbolic names, and must be included in addition to brackets that delimit a bracket expression, i.e.:
  /z[:digit:]z/ matches the strings z:z zdz zgz ziz and ztz ( but not z3z) as it specifies a bracket expression.

  [[:digit:]] is equivalent to [0123456789], i.e. imatches any single digit.
  [[:alnum:]] is frequently equivalent to [0-9A-Za-z].
  Depending on the C locale and the ASCII character encoding.
  Whereas the [:alnum:] is independent of locale and character set.

  Underscore (_) is not considered [:alpha:].

  \d (word) is a synonym for [[:digit:]]and
  \w (word) is a synonym for [[:alnum:]]and
  \W is a synonym for [^[:alnum:]].
  \s is a synonym for any white space character.

subexpression is defined using \(…\)

backreference \n, where n is a single digit, matches the substring previously matched by the n^th subexpression
Frequently on the right hand side of a substitute command

Precedence: Repetition, concatenation, alternation.
A subexpression overrides precedence rules.