grep Programs and Regular expressions

grep searches input files for lines that match a given pattern.

grep programs

'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. There are three major variants of 'grep', controlled by the following options.

        Interpret PATTERN as a basic regular expression.  This is the default.
        Interpret PATTERN as an extended regular expression.
        Interpret PATTERN as a list of fixed strings, separated by
        newlines, any of which is to be matched.

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 expressions 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 can 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 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:], [: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 can be followed by one of several repetition operators:

   ?   The preceding item is optional and will be 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 can be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.

Two regular expressions can 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 subexpression can 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 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.


An example shell command that invokes GNU 'grep':

     grep -i 'hello.*world' menu.h main.c

This lists all lines in the files 'menu.h' and 'main.c' that contain the string 'hello' followed by the string 'world'; this is because '.*' matches zero or more characters within a line. *Note Regular Expressions::. The '-i' option causes 'grep' to ignore case, causing it to match the line 'Hello, world!', which it would not otherwise match. *Note Invoking::, for more details about how to invoke 'grep'.

Some common questions and answers about 'grep' usage.

  1. How can I list just the names of matching files?

          grep -l 'main' *.c

     lists the names of all C files in the current directory whose
     contents mention 'main'.

  2. How do I search directories recursively?

          grep -r 'hello' /home/gigi

     searches for 'hello' in all files under the directory '/home/gigi'.  For more control of which files are searched, use
     'find', 'grep' and 'xargs'.  For example, the following command searches only C files:

          find /home/gigi -name '*.c' -print | xargs grep 'hello' /dev/null

  3. What if a pattern has a leading '-'?

          grep -e '--cut here--' *

     searches for all lines matching '--cut here--'.  Without '-e', 'grep' would attempt to parse '--cut here--' as a list of options.

  4. Suppose I want to search for a whole word, not a part of a word?

          grep -w 'hello' *

     searches only for instances of 'hello' that are entire words; it does not match 'Othello'.
     For more control, use '\<' and '\>' to match the start and end of words.  For example:

          grep 'hello\>' *

     searches only for words ending in 'hello', so it matches the word 'Othello'.

  5. How do I output context around the matching lines?

          grep -C 2 'hello' *

     prints two lines of context around each matching line.

  6. How do I force grep to print the name of the file?

     Append '/dev/null':

          grep 'eli' /etc/passwd /dev/null

  7. Why do people use strange regular expressions on 'ps' output?

          ps -ef | grep '[c]ron'

     If the pattern had been written without the square brackets, it would have matched not only the
     'ps' output line for 'cron', but also the 'ps' output line for 'grep'.

  8. Why does 'grep' report "Binary file matches"?

     If 'grep' listed all matching "lines" from a binary file, it would probably generate output that
     is not useful, and it might even muck up your display.  So GNU 'grep' suppresses output from files
     that appear to be binary files.  To force GNU 'grep' to output lines even from files that appear
     to be binary, use the '-a' or '--text' option.

  9. Why doesn't 'grep -lv' print nonmatching file names?

     'grep -lv' lists the names of all files containing one or more lines that do not match.
     To list the names of all files that contain no matching lines, use the '-L' or '--files-without-match'

 10. I can do OR with '|', but what about AND?

          grep 'paul' /etc/motd | grep 'franc,ois'

     finds all lines that contain both 'paul' and 'franc,ois'.

 11. How can I search in both standard input and in files?

     Use the special file name '-':

          cat /etc/passwd | grep 'alain' - /etc/motd


Large repetition counts in the '{m,n}' construct can cause 'grep' to use lots of memory. In addition, certain other obscure regular expressions require exponential time and space, and might cause grep to run out of memory. Backreferences are very slow, and can require exponential time.

"I stand for freedom of expression, doing what you believe in, and going after your dreams" ~ Madonna Ciccone

Related linux commands

egrep - Search file(s) for lines that match an extended expression.
fgrep - Search file(s) for lines that match a fixed string.
gawk - 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.
Equivalent Windows command: FINDSTR - Search for strings in files.

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