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<TITLE>BBS水木清华站∶精华区</TITLE></HEAD><BODY><CENTER><H1>BBS水木清华站∶精华区</H1></CENTER>发信人:&nbsp;starw&nbsp;(化缘道人),&nbsp;信区:&nbsp;Linux&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<BR>标&nbsp;&nbsp;题:&nbsp;Python&nbsp;Regular&nbsp;Expression&nbsp;HOWTO&nbsp;4.4&nbsp;<BR>发信站:&nbsp;BBS&nbsp;水木清华站&nbsp;(Tue&nbsp;Nov&nbsp;21&nbsp;23:48:29&nbsp;2000)&nbsp;<BR>&nbsp;<BR>4.4&nbsp;Other&nbsp;Assertions&nbsp;&nbsp;<BR>&nbsp;<BR>Another&nbsp;zero-width&nbsp;assertion&nbsp;is&nbsp;the&nbsp;lookahead&nbsp;assertion.&nbsp;Lookahead&nbsp;assertions&nbsp;&nbsp;<BR>are&nbsp;available&nbsp;in&nbsp;both&nbsp;positive&nbsp;and&nbsp;negative&nbsp;form,&nbsp;and&nbsp;look&nbsp;like&nbsp;this:&nbsp;&nbsp;<BR>&nbsp;<BR>(?=...)&nbsp;&nbsp;<BR>&nbsp;<BR>Positive&nbsp;lookahead&nbsp;assertion.&nbsp;This&nbsp;succeeds&nbsp;if&nbsp;the&nbsp;contained&nbsp;regular&nbsp;&nbsp;<BR>expression,&nbsp;represented&nbsp;here&nbsp;by&nbsp;...,&nbsp;successfully&nbsp;matches&nbsp;at&nbsp;the&nbsp;current&nbsp;&nbsp;<BR>location,&nbsp;and&nbsp;fails&nbsp;otherwise.&nbsp;But,&nbsp;once&nbsp;the&nbsp;contained&nbsp;expression&nbsp;has&nbsp;been&nbsp;&nbsp;<BR>tried,&nbsp;the&nbsp;matching&nbsp;engine&nbsp;doesn't&nbsp;advance&nbsp;at&nbsp;all;&nbsp;the&nbsp;rest&nbsp;of&nbsp;the&nbsp;pattern&nbsp;&nbsp;<BR>is&nbsp;tried&nbsp;right&nbsp;where&nbsp;the&nbsp;assertion&nbsp;started.&nbsp;&nbsp;<BR>&nbsp;<BR>(?!...)&nbsp;&nbsp;<BR>&nbsp;<BR>Negative&nbsp;lookahead&nbsp;assertion.&nbsp;This&nbsp;is&nbsp;the&nbsp;opposite&nbsp;of&nbsp;the&nbsp;positive&nbsp;assertion;&nbsp;&nbsp;<BR>it&nbsp;succeeds&nbsp;if&nbsp;the&nbsp;contained&nbsp;expression&nbsp;doesn't&nbsp;match&nbsp;at&nbsp;the&nbsp;current&nbsp;position&nbsp;&nbsp;<BR>in&nbsp;the&nbsp;string.&nbsp;An&nbsp;example&nbsp;will&nbsp;help&nbsp;make&nbsp;this&nbsp;concrete,&nbsp;and&nbsp;will&nbsp;demonstrate&nbsp;&nbsp;<BR>a&nbsp;case&nbsp;where&nbsp;a&nbsp;lookahead&nbsp;is&nbsp;useful.&nbsp;Consider&nbsp;a&nbsp;simple&nbsp;pattern&nbsp;to&nbsp;match&nbsp;a&nbsp;&nbsp;<BR>filename,&nbsp;and&nbsp;split&nbsp;it&nbsp;apart&nbsp;into&nbsp;a&nbsp;base&nbsp;name&nbsp;and&nbsp;an&nbsp;extension,&nbsp;separated&nbsp;by&nbsp;&nbsp;<BR>a&nbsp;&quot;.&quot;.&nbsp;For&nbsp;example,&nbsp;in&nbsp;&quot;news.rc&quot;,&nbsp;&quot;news&quot;is&nbsp;the&nbsp;base&nbsp;name,&nbsp;and&nbsp;&quot;rc&quot;&nbsp;is&nbsp;the&nbsp;&nbsp;<BR>filename's&nbsp;extension.&nbsp;&nbsp;<BR>&nbsp;<BR>The&nbsp;pattern&nbsp;to&nbsp;match&nbsp;this&nbsp;is&nbsp;quite&nbsp;simple:&nbsp;.*[.].*$.&nbsp;(Notice&nbsp;that&nbsp;the&nbsp;&quot;.&quot;&nbsp;&nbsp;<BR>needs&nbsp;to&nbsp;be&nbsp;treated&nbsp;specially&nbsp;because&nbsp;it's&nbsp;a&nbsp;metacharacter;&nbsp;I've&nbsp;put&nbsp;it&nbsp;&nbsp;<BR>inside&nbsp;a&nbsp;character&nbsp;class.&nbsp;Also&nbsp;notice&nbsp;the&nbsp;trailing&nbsp;$;&nbsp;this&nbsp;is&nbsp;added&nbsp;to&nbsp;ensure&nbsp;&nbsp;<BR>that&nbsp;all&nbsp;the&nbsp;rest&nbsp;of&nbsp;the&nbsp;string&nbsp;must&nbsp;be&nbsp;included&nbsp;in&nbsp;the&nbsp;extension.)&nbsp;This&nbsp;&nbsp;<BR>regular&nbsp;expression&nbsp;matches&nbsp;&quot;foo.bar&quot;&nbsp;and&nbsp;&quot;autoexec.bat&quot;&nbsp;and&nbsp;&quot;sendmail.cf&quot;&nbsp;&nbsp;<BR>and&nbsp;&quot;printers.conf&quot;.&nbsp;&nbsp;<BR>&nbsp;<BR>Now,&nbsp;consider&nbsp;complicating&nbsp;the&nbsp;problem&nbsp;a&nbsp;bit;&nbsp;what&nbsp;if&nbsp;you&nbsp;want&nbsp;to&nbsp;match&nbsp;&nbsp;<BR>filenames&nbsp;where&nbsp;the&nbsp;extension&nbsp;is&nbsp;not&nbsp;&quot;bat&quot;?&nbsp;Some&nbsp;incorrect&nbsp;attempts:&nbsp;&nbsp;<BR>&nbsp;<BR>.*[.][^b].*$&nbsp;&nbsp;<BR>&nbsp;<BR>First&nbsp;attempt:&nbsp;Exclude&nbsp;&quot;bat&quot;&nbsp;by&nbsp;requiring&nbsp;that&nbsp;the&nbsp;first&nbsp;character&nbsp;of&nbsp;the&nbsp;&nbsp;<BR>extension&nbsp;is&nbsp;not&nbsp;a&nbsp;&quot;b&quot;.&nbsp;This&nbsp;is&nbsp;wrong,&nbsp;because&nbsp;it&nbsp;also&nbsp;doesn't&nbsp;match&nbsp;&quot;foo.bar&quot;.&nbsp;&nbsp;<BR>&nbsp;<BR>.*[.]([^b]..|.[^a].|..[^t])$&nbsp;&nbsp;<BR>&nbsp;<BR>The&nbsp;expression&nbsp;gets&nbsp;messier&nbsp;when&nbsp;you&nbsp;try&nbsp;to&nbsp;patch&nbsp;up&nbsp;the&nbsp;first&nbsp;solution&nbsp;by&nbsp;&nbsp;<BR>requiring&nbsp;one&nbsp;of&nbsp;the&nbsp;following&nbsp;cases&nbsp;must&nbsp;match:&nbsp;the&nbsp;first&nbsp;character&nbsp;of&nbsp;the&nbsp;&nbsp;<BR>extension&nbsp;isn't&nbsp;&quot;b&quot;;&nbsp;the&nbsp;second&nbsp;character&nbsp;isn't&nbsp;&quot;a&quot;;&nbsp;or&nbsp;the&nbsp;third&nbsp;character&nbsp;&nbsp;<BR>isn't&nbsp;&quot;t&quot;.&nbsp;This&nbsp;accepts&nbsp;&quot;foo.bar&quot;&nbsp;and&nbsp;rejects&nbsp;&quot;autoexec.bat&quot;,&nbsp;but&nbsp;it&nbsp;requires&nbsp;&nbsp;<BR>a&nbsp;three-letter&nbsp;extension,&nbsp;and&nbsp;doesn't&nbsp;accept&nbsp;&quot;sendmail.cf&quot;.&nbsp;Another&nbsp;bug,&nbsp;so&nbsp;&nbsp;<BR>we'll&nbsp;complicate&nbsp;the&nbsp;pattern&nbsp;again&nbsp;in&nbsp;an&nbsp;effort&nbsp;to&nbsp;fix&nbsp;it.&nbsp;&nbsp;<BR>&nbsp;<BR>.*[.]([^b].?.?|.[^a]?.?|..?[^t]?)$&nbsp;&nbsp;<BR>&nbsp;<BR>In&nbsp;the&nbsp;third&nbsp;attempt,&nbsp;the&nbsp;second&nbsp;and&nbsp;third&nbsp;letters&nbsp;are&nbsp;all&nbsp;made&nbsp;optional&nbsp;in&nbsp;&nbsp;<BR>order&nbsp;to&nbsp;allow&nbsp;matching&nbsp;extensions&nbsp;shorter&nbsp;than&nbsp;three&nbsp;characters,&nbsp;such&nbsp;as&nbsp;&nbsp;<BR>&quot;sendmail.cf&quot;.&nbsp;&nbsp;<BR>&nbsp;<BR>The&nbsp;pattern's&nbsp;getting&nbsp;really&nbsp;complicated&nbsp;now,&nbsp;which&nbsp;makes&nbsp;it&nbsp;hard&nbsp;to&nbsp;read&nbsp;and&nbsp;&nbsp;<BR>understand.&nbsp;When&nbsp;you&nbsp;write&nbsp;a&nbsp;regular&nbsp;expression,&nbsp;ask&nbsp;yourself:&nbsp;if&nbsp;you&nbsp;&nbsp;<BR>encountered&nbsp;this&nbsp;expression&nbsp;in&nbsp;a&nbsp;program,&nbsp;how&nbsp;hard&nbsp;would&nbsp;it&nbsp;be&nbsp;to&nbsp;figure&nbsp;out&nbsp;&nbsp;<BR>what&nbsp;the&nbsp;expression&nbsp;was&nbsp;intended&nbsp;to&nbsp;do?&nbsp;Worse,&nbsp;this&nbsp;solution&nbsp;doesn't&nbsp;scale&nbsp;&nbsp;<BR>well;&nbsp;if&nbsp;the&nbsp;problem&nbsp;changes,&nbsp;and&nbsp;you&nbsp;want&nbsp;to&nbsp;exclude&nbsp;both&nbsp;&quot;bat&quot;&nbsp;and&nbsp;&quot;exe&quot;&nbsp;as&nbsp;&nbsp;<BR>extensions,&nbsp;the&nbsp;pattern&nbsp;would&nbsp;get&nbsp;still&nbsp;more&nbsp;complicated&nbsp;and&nbsp;confusing.&nbsp;&nbsp;<BR>&nbsp;<BR>A&nbsp;negative&nbsp;lookahead&nbsp;cuts&nbsp;through&nbsp;all&nbsp;this.&nbsp;Go&nbsp;back&nbsp;to&nbsp;the&nbsp;original&nbsp;pattern,&nbsp;&nbsp;<BR>and,&nbsp;before&nbsp;the&nbsp;.*&nbsp;which&nbsp;matches&nbsp;the&nbsp;extension,&nbsp;insert&nbsp;(?!bat$).&nbsp;This&nbsp;means:&nbsp;&nbsp;<BR>if&nbsp;the&nbsp;expression&nbsp;bat&nbsp;doesn't&nbsp;match&nbsp;at&nbsp;this&nbsp;point,&nbsp;try&nbsp;the&nbsp;rest&nbsp;of&nbsp;the&nbsp;&nbsp;<BR>pattern;&nbsp;if&nbsp;bat$&nbsp;does&nbsp;match,&nbsp;the&nbsp;whole&nbsp;pattern&nbsp;will&nbsp;fail.&nbsp;(The&nbsp;trailing&nbsp;$&nbsp;is&nbsp;&nbsp;<BR>required&nbsp;to&nbsp;ensure&nbsp;that&nbsp;something&nbsp;like&nbsp;&quot;sample.batch&quot;,&nbsp;where&nbsp;the&nbsp;extension&nbsp;&nbsp;<BR>only&nbsp;starts&nbsp;with&nbsp;&quot;bat&quot;,&nbsp;will&nbsp;be&nbsp;allowed.&nbsp;&nbsp;<BR>&nbsp;<BR>After&nbsp;this&nbsp;modification,&nbsp;the&nbsp;whole&nbsp;pattern&nbsp;is&nbsp;.*[.](?!bat$).*$.&nbsp;Excluding&nbsp;&nbsp;<BR>another&nbsp;filename&nbsp;extension&nbsp;is&nbsp;now&nbsp;easy;&nbsp;simply&nbsp;add&nbsp;it&nbsp;as&nbsp;an&nbsp;alternative&nbsp;&nbsp;<BR>inside&nbsp;the&nbsp;assertion.&nbsp;.*[.](?!bat$|exe$).*$&nbsp;excludes&nbsp;both&nbsp;&quot;bat&quot;&nbsp;and&nbsp;&quot;exe&quot;.&nbsp;&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;<BR>--&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;铜铁投洪冶，蝼蚁上粉墙。&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;阴阳无二义，天地我中央。&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>※&nbsp;来源:·BBS&nbsp;水木清华站&nbsp;smth.org·[FROM:&nbsp;202.117.27.35]&nbsp;<BR><CENTER><H1>BBS水木清华站∶精华区</H1></CENTER></BODY></HTML>
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