Editing Grep (section)

==Implementations==
A variety of grep implementations are available in many operating systems and software development environments.<ref>{{cite tech report |first=Tony |last=Abou-Assaleh |author2=Wei Ai|title=Survey of Global Regular Expression Print (GREP) Tools |institution=Dalhousie University|date=March 2004}}</ref> Early variants included egrep and fgrep, introduced in [[Version 7 Unix]].{{r|reader}} The egrep variant supports an [[Regular expression#POSIX basic and extended|extended regular expression]] syntax added by [[Alfred Aho]] after [[Ken Thompson]]'s original regular expression implementation.<ref name=Huma1988>{{cite journal|last1=Hume|first1=Andrew|title=A Tale of Two Greps|journal=Software: Practice and Experience|date=1988|volume=18|issue=11|page=1063|doi=10.1002/spe.4380181105|s2cid=6395770}}</ref> The "fgrep" variant searches for any of a list of ''fixed'' strings using the [[Aho–Corasick string matching algorithm]].<ref name=Meurant1990>{{cite book|last1=Meurant|first1=Gerard|title=Algorithms and Complexity|date=12 Sep 1990|publisher=Elsevier Science|page=278|isbn=9780080933917|url=https://books.google.com/books?id=6WriBQAAQBAJ|access-date=12 December 2015|url-status=live|archive-url=https://web.archive.org/web/20160304084311/https://books.google.com/books?id=6WriBQAAQBAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0|archive-date=4 March 2016}}</ref> Binaries of these variants exist in modern systems, usually linking to grep or calling grep as a shell script with the appropriate flag added, e.g. <code>exec grep -E "$@"</code>. egrep and fgrep, while commonly deployed on POSIX systems, to the point the POSIX specification mentions their widespread existence, are actually not part of POSIX.<ref>{{cite web|title=grep|url=http://pubs.opengroup.org/onlinepubs/009695399/utilities/grep.html|website=www.pubs.opengroup.org|publisher=The Open Group|access-date=12 December 2015|url-status=live|archive-url=https://web.archive.org/web/20151128184349/http://pubs.opengroup.org/onlinepubs/009695399/utilities/grep.html|archive-date=28 November 2015}}</ref>

Other commands contain the word "grep" to indicate they are search tools, typically ones that rely on regular expression matches. The [[pgrep]] utility, for instance, displays the processes whose names match a given regular expression.<ref>{{cite web|title=pgrep(1)|url=http://linux.die.net/man/1/pgrep|website=www.linux.die.net|access-date=12 December 2015|url-status=live|archive-url=https://web.archive.org/web/20151222084135/http://linux.die.net/man/1/pgrep|archive-date=22 December 2015}}</ref>

In the [[Perl]] programming language, <code>grep</code> is a built-in function that finds elements in a list that satisfy a certain property.<ref>{{cite web|title=grep|url=http://perldoc.perl.org/functions/grep.html|website=www.perldoc.perl.org|access-date=12 December 2015|url-status=live|archive-url=https://web.archive.org/web/20151207062445/http://perldoc.perl.org/functions/grep.html|archive-date=7 December 2015}}</ref> This [[higher-order function]] is typically named <code>[[filter (higher-order function)|filter]]</code> or <code>where</code> in other languages.

{{anchor|pcregrep}}The pcregrep command is an implementation of grep that uses [[Regular expression#Perl and PCRE|Perl regular expression]] syntax.<ref>{{cite web|title=pcregrep man page|url=http://www.pcre.org/original/doc/html/pcregrep.html|website=www.pcre.org|publisher=University of Cambridge|access-date=12 December 2015|url-status=live|archive-url=https://web.archive.org/web/20151223035259/http://www.pcre.org/original/doc/html/pcregrep.html|archive-date=23 December 2015}}</ref> Similar functionality can be invoked in the GNU version of grep with the <code>-P</code> flag.<ref>{{cite web|title=grep(1)|url=http://linux.die.net/man/1/grep|website=www.linux.die.net|access-date=12 December 2015|url-status=live|archive-url=https://web.archive.org/web/20151210004321/http://linux.die.net/man/1/grep|archive-date=10 December 2015}}</ref>

[[Porting|Ports]] of grep (within [[Cygwin]] and [[GnuWin32]], for example) also run under [[Microsoft Windows]]. Some versions of Windows feature the similar qgrep or [[findstr]] command.<ref>{{cite book
| last       = Spalding
| first      = George
| title      = Windows 2000 administration
| url        = https://archive.org/details/windows2000admin0000spal
| url-access = registration
| access-date = 2010-12-10
| series    = Network professional's library
| year      = 2000
| publisher = Osborne/McGraw-Hill
| isbn      = 978-0-07-882582-8
| pages     = [https://archive.org/details/windows2000admin0000spal/page/634 634]
| quote     = QGREP.EXE[:] A similar tool to grep in UNIX, this tool can be used to search for a text string
}}</ref>

A grep command is also part of [[ASCII Corporation|ASCII]]'s ''MSX-DOS2 Tools'' for [[MSX-DOS]] version 2.<ref>{{cite web| url = https://archive.org/details/MSXDOS2TOOLS| title = MSX-DOS2 Tools User's Manual by ASCII Corporation| date = April 1993}}</ref>

The grep, egrep, and fgrep commands have also been ported to the [[IBM i]] operating system.<ref>{{cite web |title=IBM System i Version 7.2 Programming Qshell |language=en |author=IBM |website=[[IBM]] |author-link=IBM |url=https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/rzahz/rzahzpdf.pdf?view=kc |access-date=2020-09-05 }}</ref>

The software [[Adobe InDesign]] has functions GREP (since CS3 version (2007)<ref>{{cite web|url=https://creativepro.com/review-adobe-indesign-cs3/|title=Review: Adobe InDesign CS3 - CreativePro.com|date=20 April 2007|website=creativepro.com|access-date=24 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180105233709/https://creativepro.com/review-adobe-indesign-cs3/|archive-date=5 January 2018}}</ref>), in the ''find/change'' dialog box<ref>{{cite web|title=InDesign Help: find/change|url=https://helpx.adobe.com/indesign/using/find-change.html|access-date=2016-08-12|url-status=live|archive-url=https://web.archive.org/web/20160828124223/https://helpx.adobe.com/indesign/using/find-change.html|archive-date=2016-08-28}}</ref> "GREP" tab, and introduced with InDesign CS4<ref>{{cite web |url=http://carijansen.com/tip-088/ |title=InDesign: GREP Styles (1) Setting text between parentheses in Italic |access-date=2018-01-05 |url-status=live |archive-url=https://web.archive.org/web/20170924230421/http://carijansen.com/tip-088/ |archive-date=2017-09-24 }}</ref> in ''paragraph styles''<ref>{{cite web|title=InDesign Help: GREP styles|url=https://helpx.adobe.com/indesign/using/drop-caps-nested-styles.html#create_grep_styles|access-date=2016-08-12|url-status=live|archive-url=https://web.archive.org/web/20160828114627/https://helpx.adobe.com/indesign/using/drop-caps-nested-styles.html#create_grep_styles|archive-date=2016-08-28}}</ref> "GREP styles".

===agrep===
{{main|agrep}}
'''agrep''' (approximate grep) is an [[open-source software|open-source]] [[approximate string matching]] program, developed by [[Udi Manber]] and Sun Wu between 1988 and 1991,<ref>{{cite conference |title=Agrep -- a fast approximate pattern-matching tool |last1=Wu |first1=Sun |last2=Manber |first2=Udi |date=20–24 January 1992 |location=San Francisco, California |conference=1992 Winter USENIX Conference |citeseerx = 10.1.1.89.5424}}</ref> for use with the [[Unix]] operating system. It was later ported to [[OS/2]], [[DOS]], and [[Microsoft Windows|Windows]].

''a''grep matches even when the text only ''approximately'' fits the search pattern.<ref name=eGrep.SunX>{{cite magazine |magazine=Sun Expert|author=S. Lee Henry |date=June 1998 |pages=35–26|title=Proper Searching}}</ref>

This following invocation finds <code>netmasks</code> in file <code>myfile</code>, but also any other word that can be derived from it, given no more than two substitutions.
<pre>agrep -2 netmasks myfile</pre>
This example generates a list of matches with the closest, that is those with the fewest, substitutions listed first. The command flag <code>-B</code> means "best":
<pre>agrep -B netmasks myfile</pre>