GREP - Manpage

Quick Examples

grep -E -i -w 'vivek|raj' /etc/passwd

grep --help

Usage: grep [OPTION]... PATTERNS [FILE]...
Search for PATTERNS in each FILE.
Example: grep -i 'hello world' menu.h main.c
PATTERNS can contain multiple patterns separated by newlines.

Pattern selection and interpretation:
  -E, --extended-regexp     PATTERNS are extended regular expressions
  -F, --fixed-strings       PATTERNS are strings
  -G, --basic-regexp        PATTERNS are basic regular expressions
  -P, --perl-regexp         PATTERNS are Perl regular expressions
  -e, --regexp=PATTERNS     use PATTERNS for matching
  -f, --file=FILE           take PATTERNS from FILE
  -i, --ignore-case         ignore case distinctions in patterns and data
      --no-ignore-case      do not ignore case distinctions (default)
  -w, --word-regexp         match only whole words
  -x, --line-regexp         match only whole lines
  -z, --null-data           a data line ends in 0 byte, not newline

Miscellaneous:
  -s, --no-messages         suppress error messages
  -v, --invert-match        select non-matching lines
  -V, --version             display version information and exit
      --help                display this help text and exit

Output control:
  -m, --max-count=NUM       stop after NUM selected lines
  -b, --byte-offset         print the byte offset with output lines
  -n, --line-number         print line number with output lines
      --line-buffered       flush output on every line
  -H, --with-filename       print file name with output lines
  -h, --no-filename         suppress the file name prefix on output
      --label=LABEL         use LABEL as the standard input file name prefix
  -o, --only-matching       show only nonempty parts of lines that match
  -q, --quiet, --silent     suppress all normal output
      --binary-files=TYPE   assume that binary files are TYPE;
                            TYPE is 'binary', 'text', or 'without-match'
  -a, --text                equivalent to --binary-files=text
  -I                        equivalent to --binary-files=without-match
  -d, --directories=ACTION  how to handle directories;
                            ACTION is 'read', 'recurse', or 'skip'
  -D, --devices=ACTION      how to handle devices, FIFOs and sockets;
                            ACTION is 'read' or 'skip'
  -r, --recursive           like --directories=recurse
  -R, --dereference-recursive  likewise, but follow all symlinks
      --include=GLOB        search only files that match GLOB (a file pattern)
      --exclude=GLOB        skip files that match GLOB
      --exclude-from=FILE   skip files that match any file pattern from FILE
      --exclude-dir=GLOB    skip directories that match GLOB
  -L, --files-without-match  print only names of FILEs with no selected lines
  -l, --files-with-matches  print only names of FILEs with selected lines
  -c, --count               print only a count of selected lines per FILE
  -T, --initial-tab         make tabs line up (if needed)
  -Z, --null                print 0 byte after FILE name

Context control:
  -B, --before-context=NUM  print NUM lines of leading context
  -A, --after-context=NUM   print NUM lines of trailing context
  -C, --context=NUM         print NUM lines of output context
  -NUM                      same as --context=NUM
      --group-separator=SEP  print SEP on line between matches with context
      --no-group-separator  do not print separator for matches with context
      --color[=WHEN],
      --colour[=WHEN]       use markers to highlight the matching strings;
                            WHEN is 'always', 'never', or 'auto'
  -U, --binary              do not strip CR characters at EOL (MSDOS/Windows)

When FILE is '-', read standard input.  With no FILE, read '.' if
recursive, '-' otherwise.  With fewer than two FILEs, assume -h.
Exit status is 0 if any line is selected, 1 otherwise;
if any error occurs and -q is not given, the exit status is 2.

Report bugs to: bug-grep@gnu.org
GNU grep home page: <https://www.gnu.org/software/grep/>
General help using GNU software: <https://www.gnu.org/gethelp/>

Manpage

GREP(1)                                                                                         User Commands                                                                                         GREP(1)

NAME
       grep, egrep, fgrep, rgrep - print lines that match patterns

SYNOPSIS
       grep [OPTION...] PATTERNS [FILE...]
       grep [OPTION...] -e PATTERNS ... [FILE...]
       grep [OPTION...] -f PATTERN_FILE ... [FILE...]

DESCRIPTION
       grep  searches  for  PATTERNS  in each FILE.  PATTERNS is one or more patterns separated by newline characters, and grep prints each line that matches a pattern.  Typically PATTERNS should be quoted
       when grep is used in a shell command.

       A FILE of “-” stands for standard input.  If no FILE is given, recursive searches examine the working directory, and nonrecursive searches read standard input.

       Debian also includes the variant programs egrep, fgrep and rgrep.  These programs are the same as grep -E, grep -F, and grep -r, respectively.  These variants are  deprecated  upstream,  but  Debian
       provides for backward compatibility. For portability reasons, it is recommended to avoid the variant programs, and use grep with the related option instead.

OPTIONS
   Generic Program Information
       --help Output a usage message and exit.

       -V, --version
              Output the version number of grep and exit.

   Pattern Syntax
       -E, --extended-regexp
              Interpret PATTERNS as extended regular expressions (EREs, see below).

       -F, --fixed-strings
              Interpret PATTERNS as fixed strings, not regular expressions.

       -G, --basic-regexp
              Interpret PATTERNS as basic regular expressions (BREs, see below).  This is the default.

       -P, --perl-regexp
              Interpret PATTERNS as Perl-compatible regular expressions (PCREs).  This option is experimental when combined with the -z (--null-data) option, and grep -P may warn of unimplemented features.

   Matching Control
       -e PATTERNS, --regexp=PATTERNS
              Use  PATTERNS  as the patterns.  If this option is used multiple times or is combined with the -f (--file) option, search for all patterns given.  This option can be used to protect a pattern
              beginning with “-”.

       -f FILE, --file=FILE
              Obtain patterns from FILE, one per line.  If this option is used multiple times or is combined with the -e (--regexp) option, search for all patterns given.   The  empty  file  contains  zero
              patterns, and therefore matches nothing.

       -i, --ignore-case
              Ignore case distinctions in patterns and input data, so that characters that differ only in case match each other.

       --no-ignore-case
              Do  not  ignore  case distinctions in patterns and input data.  This is the default.  This option is useful for passing to shell scripts that already use -i, to cancel its effects because the
              two options override each other.

       -v, --invert-match
              Invert the sense of matching, to select non-matching lines.

       -w, --word-regexp
              Select only those lines containing matches that form whole words.  The test is that the matching substring must either be at the beginning of the line, or preceded by a  non-word  constituent
              character.   Similarly,  it  must be either at the end of the line or followed by a non-word constituent character.  Word-constituent characters are letters, digits, and the underscore.  This
              option has no effect if -x is also specified.

       -x, --line-regexp
              Select only those matches that exactly match the whole line.  For a regular expression pattern, this is like parenthesizing the pattern and then surrounding it with ^ and $.

   General Output Control
       -c, --count
              Suppress normal output; instead print a count of matching lines for each input file.  With the -v, --invert-match option (see above), count non-matching lines.

       --color[=WHEN], --colour[=WHEN]
              Surround the matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for fields and groups of context lines) with escape  sequences
              to display them in color on the terminal.  The colors are defined by the environment variable GREP_COLORS.  WHEN is never, always, or auto.

       -L, --files-without-match
              Suppress normal output; instead print the name of each input file from which no output would normally have been printed.

       -l, --files-with-matches
              Suppress normal output; instead print the name of each input file from which output would normally have been printed.  Scanning each input file stops upon first match.

       -m NUM, --max-count=NUM
              Stop  reading  a  file after NUM matching lines.  If NUM is zero, grep stops right away without reading input.  A NUM of -1 is treated as infinity and grep does not stop; this is the default.
              If the input is standard input from a regular file, and NUM matching lines are output, grep ensures that the standard input is positioned to just after the last matching line before  exiting,
              regardless  of  the  presence  of trailing context lines.  This enables a calling process to resume a search.  When grep stops after NUM matching lines, it outputs any trailing context lines.
              When the -c or --count option is also used, grep does not output a count greater than NUM.  When the -v or --invert-match option is also used, grep stops  after  outputting  NUM  non-matching
              lines.

       -o, --only-matching
              Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.

       -q, --quiet, --silent
              Quiet; do not write anything to standard output.  Exit immediately with zero status if any match is found, even if an error was detected.  Also see the -s or --no-messages option.

       -s, --no-messages
              Suppress error messages about nonexistent or unreadable files.

   Output Line Prefix Control
       -b, --byte-offset
              Print the 0-based byte offset within the input file before each line of output.  If -o (--only-matching) is specified, print the offset of the matching part itself.

       -H, --with-filename
              Print the file name for each match.  This is the default when there is more than one file to search.  This is a GNU extension.

       -h, --no-filename
              Suppress the prefixing of file names on output.  This is the default when there is only one file (or only standard input) to search.

       --label=LABEL
              Display  input actually coming from standard input as input coming from file LABEL.  This can be useful for commands that transform a file's contents before searching, e.g., gzip -cd foo.gz |
              grep --label=foo -H 'some pattern'.  See also the -H option.

       -n, --line-number
              Prefix each line of output with the 1-based line number within its input file.

       -T, --initial-tab
              Make sure that the first character of actual line content lies on a tab stop, so that the alignment of tabs looks normal.  This is useful with options that prefix their output to  the  actual
              content:  -H,-n, and -b.  In order to improve the probability that lines from a single file will all start at the same column, this also causes the line number and byte offset (if present) to
              be printed in a minimum size field width.

       -Z, --null
              Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name.  For example, grep -lZ outputs a zero byte after each file name instead of  the  usual
              newline.   This  option makes the output unambiguous, even in the presence of file names containing unusual characters like newlines.  This option can be used with commands like find -print0,
              perl -0, sort -z, and xargs -0 to process arbitrary file names, even those that contain newline characters.

   Context Line Control
       -A NUM, --after-context=NUM
              Print NUM lines of trailing context after matching lines.  Places a line containing a group separator (--) between contiguous groups of matches.  With the -o or --only-matching  option,  this
              has no effect and a warning is given.

       -B NUM, --before-context=NUM
              Print  NUM  lines of leading context before matching lines.  Places a line containing a group separator (--) between contiguous groups of matches.  With the -o or --only-matching option, this
              has no effect and a warning is given.

       -C NUM, -NUM, --context=NUM
              Print NUM lines of output context.  Places a line containing a group separator (--) between contiguous groups of matches.  With the -o or --only-matching option, this  has  no  effect  and  a
              warning is given.

       --group-separator=SEP
              When -A, -B, or -C are in use, print SEP instead of -- between groups of lines.

       --no-group-separator
              When -A, -B, or -C are in use, do not print a separator between groups of lines.

   File and Directory Selection
       -a, --text
              Process a binary file as if it were text; this is equivalent to the --binary-files=text option.

       --binary-files=TYPE
              If  a  file's  data or metadata indicate that the file contains binary data, assume that the file is of type TYPE.  Non-text bytes indicate binary data; these are either output bytes that are
              improperly encoded for the current locale, or null input bytes when the -z option is not given.

              By default, TYPE is binary, and grep suppresses output after null input binary data is discovered, and suppresses output lines that contain improperly  encoded  data.   When  some  output  is
              suppressed, grep follows any output with a message to standard error saying that a binary file matches.

              If TYPE is without-match, when grep discovers null input binary data it assumes that the rest of the file does not match; this is equivalent to the -I option.

              If TYPE is text, grep processes a binary file as if it were text; this is equivalent to the -a option.

              When  type  is  binary, grep may treat non-text bytes as line terminators even without the -z option.  This means choosing binary versus text can affect whether a pattern matches a file.  For
              example, when type is binary the pattern q$ might match q immediately followed by a null byte, even though this is not matched when type is text.  Conversely, when type is binary the  pattern
              . (period) might not match a null byte.

              Warning:  The  -a option might output binary garbage, which can have nasty side effects if the output is a terminal and if the terminal driver interprets some of it as commands.  On the other
              hand, when reading files whose text encodings are unknown, it can be helpful to use -a or to set LC_ALL='C' in the environment, in order to find more matches even if the  matches  are  unsafe
              for direct display.

       -D ACTION, --devices=ACTION
              If an input file is a device, FIFO or socket, use ACTION to process it.  By default, ACTION is read, which means that devices are read just as if they were ordinary files.  If ACTION is skip,
              devices are silently skipped.

       -d ACTION, --directories=ACTION
              If an input file is a directory, use ACTION to process it.  By default, ACTION is read, i.e., read directories just as if  they  were  ordinary  files.   If  ACTION  is  skip,  silently  skip
              directories.  If ACTION is recurse, read all files under each directory, recursively, following symbolic links only if they are on the command line.  This is equivalent to the -r option.

       --exclude=GLOB
              Skip  any  command-line file with a name suffix that matches the pattern GLOB, using wildcard matching; a name suffix is either the whole name, or a trailing part that starts with a non-slash
              character immediately after a slash (/) in the name.  When searching recursively, skip any subfile whose base name matches GLOB; the base name is the part after the last slash.  A pattern can
              use *, ?, and [...] as wildcards, and \ to quote a wildcard or backslash character literally.

       --exclude-from=FILE
              Skip files whose base name matches any of the file-name globs read from FILE (using wildcard matching as described under --exclude).

       --exclude-dir=GLOB
              Skip  any  command-line  directory  with  a  name  suffix that matches the pattern GLOB.  When searching recursively, skip any subdirectory whose base name matches GLOB.  Ignore any redundant
              trailing slashes in GLOB.

       -I     Process a binary file as if it did not contain matching data; this is equivalent to the --binary-files=without-match option.

       --include=GLOB
              Search only files whose base name matches GLOB (using wildcard matching as described under --exclude).  If contradictory --include and --exclude options are given, the last matching one wins.
              If no --include or --exclude options match, a file is included unless the first such option is --include.

       -r, --recursive
              Read  all  files under each directory, recursively, following symbolic links only if they are on the command line.  Note that if no file operand is given, grep searches the working directory.
              This is equivalent to the -d recurse option.

       -R, --dereference-recursive
              Read all files under each directory, recursively.  Follow all symbolic links, unlike -r.

   Other Options
       --line-buffered
              Use line buffering on output.  This can cause a performance penalty.

       -U, --binary
              Treat the file(s) as binary.  By default, under MS-DOS and MS-Windows, grep guesses whether a file is text or binary as described for the --binary-files option.  If grep decides the file is a
              text file, it strips the CR characters from the original file contents (to make regular expressions with ^ and $ work correctly).  Specifying -U overrules this guesswork, causing all files to
              be read and passed to the matching mechanism verbatim; if the file is a text file with CR/LF pairs at the end of each line, this will cause some regular expressions to fail.  This option  has
              no effect on platforms other than MS-DOS and MS-Windows.

       -z, --null-data
              Treat  input  and output data as sequences of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline.  Like the -Z or --null option, this option can be used with
              commands like sort -z to process arbitrary file names.

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” (BRE), “extended” (ERE) and “perl” (PCRE).  In GNU grep there is no difference in available functionality between
       basic and extended syntax.  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-compatible regular expressions give additional functionality, and are documented in pcre2syntax(3) and pcre2pattern(3), but work only if PCRE support is
       enabled.

       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.  It is unspecified whether it matches an encoding error.

   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; it is unspecified whether it matches an encoding error.  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:], [:blank:], [: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 \?, \+, \{, \|, \(, and \).

EXIT STATUS
       Normally  the  exit  status  is  0 if a line is selected, 1 if no lines were selected, and 2 if an error occurred.  However, if the -q or --quiet or --silent is used and a line is selected, the exit
       status is 0 even if an error occurred.

ENVIRONMENT
       The behavior of grep is affected by the following environment variables.

       The locale for category LC_foo is specified by examining the three environment variables LC_ALL, LC_foo, LANG, in that order.  The first of these variables that is set  specifies  the  locale.   For
       example,  if  LC_ALL  is  not  set,  but  LC_MESSAGES  is  set to pt_BR, then the Brazilian Portuguese locale is used for the LC_MESSAGES category.  The C locale is used if none of these environment
       variables are set, if the locale catalog is not installed, or if grep was not compiled with national language support (NLS).  The shell command locale -a lists locales that are currently available.

       GREP_COLORS
              Controls how the --color option highlights output.  Its value is a colon-separated list of capabilities that defaults to ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36 with the rv  and  ne
              boolean capabilities omitted (i.e., false).  Supported capabilities are as follows.

              sl=    SGR  substring  for  whole  selected  lines  (i.e.,  matching  lines when the -v command-line option is omitted, or non-matching lines when -v is specified).  If however the boolean rv
                     capability and the -v command-line option are both specified, it applies to context matching lines instead.  The default is empty (i.e., the terminal's default color pair).

              cx=    SGR substring for whole context lines (i.e., non-matching lines when the -v command-line option is omitted, or matching lines  when  -v  is  specified).   If  however  the  boolean  rv
                     capability and the -v command-line option are both specified, it applies to selected non-matching lines instead.  The default is empty (i.e., the terminal's default color pair).

              rv     Boolean value that reverses (swaps) the meanings of the sl= and cx= capabilities when the -v command-line option is specified.  The default is false (i.e., the capability is omitted).

              mt=01;31
                     SGR substring for matching non-empty text in any matching line (i.e., a selected line when the -v command-line option is omitted, or a context line when -v is specified).  Setting this
                     is equivalent to setting both ms= and mc= at once to the same value.  The default is a bold red text foreground over the current line background.

              ms=01;31
                     SGR substring for matching non-empty text in a selected line.  (This is only used when the -v command-line option is omitted.)  The effect of the sl= (or cx= if rv) capability  remains
                     active when this kicks in.  The default is a bold red text foreground over the current line background.

              mc=01;31
                     SGR substring for matching non-empty text in a context line.  (This is only used when the -v command-line option is specified.)  The effect of the cx= (or sl= if rv) capability remains
                     active when this kicks in.  The default is a bold red text foreground over the current line background.

              fn=35  SGR substring for file names prefixing any content line.  The default is a magenta text foreground over the terminal's default background.

              ln=32  SGR substring for line numbers prefixing any content line.  The default is a green text foreground over the terminal's default background.

              bn=32  SGR substring for byte offsets prefixing any content line.  The default is a green text foreground over the terminal's default background.

              se=36  SGR substring for separators that are inserted between selected line fields (:), between context line fields, (-), and  between  groups  of  adjacent  lines  when  nonzero  context  is
                     specified (--).  The default is a cyan text foreground over the terminal's default background.

              ne     Boolean  value  that  prevents  clearing  to  the end of line using Erase in Line (EL) to Right (\33[K) each time a colorized item ends.  This is needed on terminals on which EL is not
                     supported.  It is otherwise useful on terminals for which the back_color_erase (bce) boolean terminfo capability does not apply, when the chosen highlight  colors  do  not  affect  the
                     background, or when EL is too slow or causes too much flicker.  The default is false (i.e., the capability is omitted).

              Note that boolean capabilities have no =... part.  They are omitted (i.e., false) by default and become true when specified.

              See  the  Select Graphic Rendition (SGR) section in the documentation of the text terminal that is used for permitted values and their meaning as character attributes.  These substring values
              are integers in decimal representation and can be concatenated with semicolons.  grep takes care of assembling  the  result  into  a  complete  SGR  sequence  (\33[...m).   Common  values  to
              concatenate  include  1  for  bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors,
              38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color  mode  background  colors,  and
              48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.

       LC_ALL, LC_COLLATE, LANG
              These variables specify the locale for the LC_COLLATE category, which determines the collating sequence used to interpret range expressions like [a-z].

       LC_ALL, LC_CTYPE, LANG
              These  variables  specify  the  locale  for the LC_CTYPE category, which determines the type of characters, e.g., which characters are whitespace.  This category also determines the character
              encoding, that is, whether text is encoded in UTF-8, ASCII, or some other encoding.  In the C or POSIX locale, all characters are encoded as a single byte and every byte is a valid character.

       LC_ALL, LC_MESSAGES, LANG
              These variables specify the locale for the LC_MESSAGES category, which determines the language that grep uses for messages.  The default C locale uses American English messages.

       POSIXLY_CORRECT
              If set, grep behaves as POSIX requires; otherwise, grep behaves more like other GNU programs.  POSIX requires that options that follow file names must be treated as file  names;  by  default,
              such options are permuted to the front of the operand list and are treated as options.  Also, POSIX requires that unrecognized options be diagnosed as “illegal”, but since they are not really
              against the law the default is to diagnose them as “invalid”.  POSIXLY_CORRECT also disables _N_GNU_nonoption_argv_flags_, described below.

       _N_GNU_nonoption_argv_flags_
              (Here N is grep's numeric process ID.)  If the ith character of this environment variable's value is 1, do not consider the ith operand of grep to be an option, even if it appears to be  one.
              A  shell  can  put  this variable in the environment for each command it runs, specifying which operands are the results of file name wildcard expansion and therefore should not be treated as
              options.  This behavior is available only with the GNU C library, and only when POSIXLY_CORRECT is not set.

NOTES
       This man page is maintained only fitfully; the full documentation is often more up-to-date.

COPYRIGHT
       Copyright 1998-2000, 2002, 2005-2022 Free Software Foundation, Inc.

       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

BUGS
   Reporting Bugs
       Email    bug    reports    to    the    bug-reporting    address    ⟨bug-grep@gnu.org⟩.     An    email    archive    ⟨https://lists.gnu.org/mailman/listinfo/bug-grep⟩    and    a    bug     tracker
       ⟨https://debbugs.gnu.org/cgi/pkgreport.cgi?package=grep⟩ are available.

   Known Bugs
       Large repetition counts in the {n,m} construct may cause grep to use lots of memory.  In addition, certain other obscure regular expressions require exponential time and space, and may cause grep to
       run out of memory.

       Back-references are very slow, and may require exponential time.

EXAMPLE
       The following example outputs the location and contents of any line containing “f” and ending in “.c”, within all files in the current directory whose names contain “g” and end in “.h”.  The -n  op‐
       tion outputs line numbers, the -- argument treats expansions of “*g*.h” starting with “-” as file names not options, and the empty file /dev/null causes file names to be output even if only one file
       name happens to be of the form “*g*.h”.

         $ grep -n -- 'f.*\.c$' *g*.h /dev/null
         argmatch.h:1:/* definitions and prototypes for argmatch.c

       The only line that matches is line 1 of argmatch.h.  Note that the regular expression syntax used in the pattern differs from the globbing syntax that the shell uses to match file names.

SEE ALSO
   Regular Manual Pages
       awk(1), cmp(1), diff(1), find(1), perl(1), sed(1), sort(1), xargs(1), read(2), pcre2(3), pcre2syntax(3), pcre2pattern(3), terminfo(5), glob(7), regex(7)

   Full Documentation
       A complete manual ⟨https://www.gnu.org/software/grep/manual/⟩ is available.  If the info and grep programs are properly installed at your site, the command

              info grep

       should give you access to the complete manual.

GNU grep 3.8                                                                                      2019-12-29                                                                                          GREP(1)

Usefull Links

• https://www.cyberciti.biz/faq/grep-regular-expressions/