Editing Regular expression (section)

===Assertions===

{| id="lookbehind" class="floatright wikitable"
|-
! Assertion !! Lookbehind !! Lookahead
|-
! Positive
| style="text-align:center;font-size:125%;"|<code>(?'''<='''{{box|inline=yes|span=yes|type=black|radius=1ex|padding=0 0.5ex|font size=80%|pattern}})</code>
| style="text-align:center;font-size:125%;"|<code>(?'''='''{{box|inline=yes|span=yes|type=black|radius=1ex|padding=0 0.5ex|font size=80%|pattern}})</code>
|-
! Negative
| style="text-align:center;font-size:125%;"|<code>(?'''&lt;!'''{{box|inline=yes|span=yes|type=black|radius=1ex|font size=80%|padding=0 0.5ex|pattern}})</code>
| style="text-align:center;font-size:125%;"|<code>(?<span style="padding:1px;">'''!'''</span>{{box|inline=yes|span=yes|type=black|radius=1ex|padding=0 0.5ex|font size=80%|pattern}})</code>
|-
| colspan="3"|Lookbehind and lookahead assertions<br/>in [[Perl]] regular expressions
|}
Other features not found in describing regular languages include assertions. These include the ubiquitous {{code|^}} and {{code|$}}, used since at least 1970,<ref>{{cite book |last1=Ritchie |first1=D. M. |last2=Thompson |first2=K. L. |title=QED Text Editor |url=http://cm.bell-labs.com/cm/cs/who/dmr/qedman.pdf |id=MM-70-1373-3 |date=June 1970 |access-date=2022-09-05 |archive-date=2015-02-03 |archive-url=https://wayback.archive-it.org/all/20150203071645/http://cm.bell-labs.com/cm/cs/who/dmr/qedman.pdf |url-status=dead}} Reprinted as "QED Text Editor Reference Manual", MHCC-004, Murray Hill Computing, Bell Laboratories (October 1972).</ref> as well as some more sophisticated extensions like lookaround that appeared in 1994.{{r|perl5}} Lookarounds define the surrounding of a match and do not spill into the match itself, a feature only relevant for the use case of string searching.{{citation needed|date=June 2023}} Some of them can be simulated in a regular language by treating the surroundings as a part of the language as well.<ref>{{cite web |author=Wandering Logic |title=How to simulate lookaheads and lookbehinds in finite state automata? |url=https://cs.stackexchange.com/a/40058 |website=Computer Science Stack Exchange |access-date=24 November 2019 |archive-date=7 October 2020 |archive-url=https://web.archive.org/web/20201007183206/https://cs.stackexchange.com/questions/2557/how-to-simulate-backreferences-lookaheads-and-lookbehinds-in-finite-state-auto/40058 |url-status=live}}</ref>

The {{visible anchor |look-ahead assertions}} {{nowrap|1=<code>(?=...)</code>}} and {{nowrap|<code>(?!...)</code>}} have been attested since at least 1994, starting with Perl 5.<ref name=perl5>{{cite web |last=Wall |first=Larry |title=Perl 5: perlre.pod |url=https://github.com/Perl/perl5/blob/a0d0e21ea6ea90a22318550944fe6cb09ae10cda/pod/perlre.pod |date=1994-10-18 |website=GitHub}}</ref> The lookbehind assertions {{nowrap|1=<code>(?<=...)</code>}} and {{nowrap|<code>(?<!...)</code>}} are attested since 1997 in a commit by Ilya Zakharevich to Perl 5.005.<ref>{{cite web |last=Zakharevich |first=Ilya |title=Jumbo Regexp Patch Applied (with Minor Fix-Up Tweaks): Perl/perl5@c277df4 |url=https://github.com/Perl/perl5/commit/c277df42229d99fecbc76f5da53793a409ac66e1 |website=GitHub |date=1997-11-19}}</ref><!-- I emailed Ilya Zakharevich to confirm, and he confirmed that he came up with the notation. -->