📄 00000003.htm

📁 一份很好的linux入门资料
💻 HTM
字号:
<HTML><HEAD>  <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;4.3&nbsp;<BR>发信站:&nbsp;BBS&nbsp;水木清华站&nbsp;(Tue&nbsp;Nov&nbsp;21&nbsp;23:45:46&nbsp;2000)&nbsp;<BR>&nbsp;<BR>嘿嘿，就一部分....看了半天才弄明白&nbsp;<BR>&nbsp;<BR>4.3&nbsp;Non-capturing,&nbsp;and&nbsp;Named&nbsp;Groups&nbsp;<BR>&nbsp;<BR>Elaborate&nbsp;REs&nbsp;may&nbsp;use&nbsp;many&nbsp;groups,&nbsp;both&nbsp;to&nbsp;capture&nbsp;substrings&nbsp;of&nbsp;interest,&nbsp;&nbsp;<BR>and&nbsp;to&nbsp;group&nbsp;and&nbsp;structure&nbsp;the&nbsp;RE&nbsp;itself.&nbsp;In&nbsp;complex&nbsp;REs,&nbsp;it&nbsp;becomes&nbsp;&nbsp;<BR>difficult&nbsp;to&nbsp;keep&nbsp;track&nbsp;of&nbsp;the&nbsp;group&nbsp;numbers.&nbsp;There&nbsp;are&nbsp;two&nbsp;features&nbsp;which&nbsp;&nbsp;<BR>help&nbsp;with&nbsp;this&nbsp;problem.&nbsp;Both&nbsp;of&nbsp;them&nbsp;use&nbsp;a&nbsp;common&nbsp;syntax&nbsp;for&nbsp;regular&nbsp;&nbsp;<BR>expression&nbsp;extensions,&nbsp;so&nbsp;we'll&nbsp;look&nbsp;at&nbsp;that&nbsp;first.&nbsp;&nbsp;<BR>&nbsp;<BR>Perl&nbsp;5&nbsp;added&nbsp;several&nbsp;additional&nbsp;features&nbsp;to&nbsp;standard&nbsp;regular&nbsp;expressions,&nbsp;&nbsp;<BR>and&nbsp;the&nbsp;Python&nbsp;re&nbsp;module&nbsp;supports&nbsp;most&nbsp;of&nbsp;them.&nbsp;It&nbsp;would&nbsp;have&nbsp;been&nbsp;difficult&nbsp;&nbsp;<BR>to&nbsp;choose&nbsp;new&nbsp;single-keystroke&nbsp;metacharacters&nbsp;or&nbsp;new&nbsp;special&nbsp;sequences&nbsp;&nbsp;<BR>beginning&nbsp;with&nbsp;&quot;\&quot;&nbsp;to&nbsp;represent&nbsp;the&nbsp;new&nbsp;features,&nbsp;without&nbsp;making&nbsp;Perl's&nbsp;&nbsp;<BR>regular&nbsp;expressions&nbsp;confusingly&nbsp;different&nbsp;from&nbsp;standard&nbsp;REs.&nbsp;If&nbsp;you&nbsp;chose&nbsp;&nbsp;<BR>&quot;&amp;&quot;&nbsp;as&nbsp;a&nbsp;new&nbsp;metacharacter,&nbsp;for&nbsp;example,&nbsp;old&nbsp;expressions&nbsp;would&nbsp;be&nbsp;assuming&nbsp;&nbsp;<BR>that&nbsp;&quot;&amp;&quot;&nbsp;was&nbsp;a&nbsp;regular&nbsp;character&nbsp;and&nbsp;wouldn't&nbsp;have&nbsp;escaped&nbsp;it&nbsp;by&nbsp;writing&nbsp;\&amp;&nbsp;&nbsp;<BR>or&nbsp;[&amp;].&nbsp;The&nbsp;solution&nbsp;chosen&nbsp;was&nbsp;to&nbsp;use&nbsp;(?...)&nbsp;as&nbsp;the&nbsp;extension&nbsp;syntax.&nbsp;&quot;?&quot;&nbsp;&nbsp;<BR>immediately&nbsp;after&nbsp;a&nbsp;parenthesis&nbsp;was&nbsp;a&nbsp;syntax&nbsp;error,&nbsp;because&nbsp;the&nbsp;&quot;?&quot;&nbsp;would&nbsp;&nbsp;<BR>have&nbsp;nothing&nbsp;to&nbsp;repeat,&nbsp;so&nbsp;this&nbsp;doesn't&nbsp;introduce&nbsp;any&nbsp;compatibility&nbsp;problems.&nbsp;&nbsp;<BR>The&nbsp;characters&nbsp;immediately&nbsp;after&nbsp;the&nbsp;&quot;?&quot;&nbsp;indicate&nbsp;what&nbsp;extension&nbsp;is&nbsp;being&nbsp;&nbsp;<BR>used,&nbsp;so&nbsp;(?=foo)&nbsp;is&nbsp;one&nbsp;thing&nbsp;(a&nbsp;positive&nbsp;lookahead&nbsp;assertion)&nbsp;and&nbsp;(?:foo)&nbsp;&nbsp;<BR>is&nbsp;something&nbsp;else&nbsp;(a&nbsp;non-capturing&nbsp;group&nbsp;containing&nbsp;the&nbsp;subexpression&nbsp;foo).&nbsp;&nbsp;<BR>&nbsp;<BR>Python&nbsp;adds&nbsp;an&nbsp;extension&nbsp;syntax&nbsp;to&nbsp;Perl's&nbsp;extension&nbsp;syntax.&nbsp;If&nbsp;the&nbsp;first&nbsp;&nbsp;<BR>character&nbsp;after&nbsp;the&nbsp;question&nbsp;mark&nbsp;is&nbsp;a&nbsp;&quot;P&quot;,&nbsp;you&nbsp;know&nbsp;that&nbsp;it's&nbsp;a&nbsp;extension&nbsp;&nbsp;<BR>that's&nbsp;specific&nbsp;to&nbsp;Python.&nbsp;Currently&nbsp;there&nbsp;are&nbsp;two&nbsp;such&nbsp;extensions:&nbsp;&nbsp;<BR>(?P&lt;name&gt;...)&nbsp;defines&nbsp;a&nbsp;named&nbsp;group,&nbsp;and&nbsp;(?P=name)&nbsp;is&nbsp;a&nbsp;backreference&nbsp;to&nbsp;a&nbsp;&nbsp;<BR>named&nbsp;group.&nbsp;If&nbsp;future&nbsp;versions&nbsp;of&nbsp;Perl&nbsp;5&nbsp;add&nbsp;similar&nbsp;features&nbsp;using&nbsp;a&nbsp;&nbsp;<BR>different&nbsp;syntax,&nbsp;the&nbsp;re&nbsp;module&nbsp;will&nbsp;be&nbsp;changed&nbsp;to&nbsp;support&nbsp;the&nbsp;new&nbsp;syntax,&nbsp;&nbsp;<BR>while&nbsp;preserving&nbsp;the&nbsp;Python-specific&nbsp;syntax&nbsp;for&nbsp;compatibility's&nbsp;sake.&nbsp;&nbsp;<BR>&nbsp;<BR>Now&nbsp;that&nbsp;we've&nbsp;looked&nbsp;at&nbsp;the&nbsp;general&nbsp;extension&nbsp;syntax,&nbsp;we&nbsp;can&nbsp;return&nbsp;to&nbsp;the&nbsp;&nbsp;<BR>features&nbsp;that&nbsp;simplify&nbsp;working&nbsp;with&nbsp;groups&nbsp;in&nbsp;complex&nbsp;REs.&nbsp;Since&nbsp;groups&nbsp;are&nbsp;&nbsp;<BR>numbered&nbsp;from&nbsp;left&nbsp;to&nbsp;right,&nbsp;and&nbsp;a&nbsp;complex&nbsp;expression&nbsp;may&nbsp;use&nbsp;many&nbsp;groups,&nbsp;&nbsp;<BR>it&nbsp;can&nbsp;become&nbsp;difficult&nbsp;to&nbsp;keep&nbsp;track&nbsp;of&nbsp;the&nbsp;correct&nbsp;numbering,&nbsp;and&nbsp;modifying&nbsp;&nbsp;<BR>such&nbsp;a&nbsp;complex&nbsp;RE&nbsp;is&nbsp;annoying.&nbsp;Insert&nbsp;a&nbsp;new&nbsp;group&nbsp;near&nbsp;the&nbsp;beginning,&nbsp;and&nbsp;you&nbsp;&nbsp;<BR>change&nbsp;the&nbsp;numbers&nbsp;of&nbsp;everything&nbsp;that&nbsp;follows&nbsp;it.&nbsp;&nbsp;<BR>&nbsp;<BR>First,&nbsp;sometimes&nbsp;you'll&nbsp;want&nbsp;to&nbsp;use&nbsp;a&nbsp;group&nbsp;to&nbsp;collect&nbsp;a&nbsp;part&nbsp;of&nbsp;a&nbsp;regular&nbsp;&nbsp;<BR>expression,&nbsp;but&nbsp;aren't&nbsp;interested&nbsp;in&nbsp;retrieving&nbsp;the&nbsp;group's&nbsp;contents.&nbsp;You&nbsp;can&nbsp;&nbsp;<BR>make&nbsp;this&nbsp;fact&nbsp;explicit&nbsp;by&nbsp;using&nbsp;a&nbsp;non-capturing&nbsp;group:&nbsp;(?:...),&nbsp;where&nbsp;you&nbsp;&nbsp;<BR>can&nbsp;put&nbsp;any&nbsp;other&nbsp;regular&nbsp;expression&nbsp;inside&nbsp;the&nbsp;parentheses.&nbsp;&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>＞<I>&gt;&gt;&nbsp;m&nbsp;=&nbsp;re.match(&quot;([abc])+'',&nbsp;&quot;abc&quot;)&nbsp;</I><BR>＞<I>&gt;&gt;&nbsp;m.groups()&nbsp;</I><BR>('c',)&nbsp;<BR>＞<I>&gt;&gt;&nbsp;m&nbsp;=&nbsp;re.match(&quot;(?:[abc])+&quot;,&nbsp;&quot;abc&quot;)&nbsp;</I><BR>＞<I>&gt;&gt;&nbsp;m.groups()&nbsp;</I><BR>()&nbsp;<BR>&nbsp;<BR>Except&nbsp;for&nbsp;the&nbsp;fact&nbsp;that&nbsp;you&nbsp;can't&nbsp;retrieve&nbsp;the&nbsp;contents&nbsp;of&nbsp;what&nbsp;the&nbsp;group&nbsp;&nbsp;<BR>matched,&nbsp;a&nbsp;non-capturing&nbsp;group&nbsp;behaves&nbsp;exactly&nbsp;the&nbsp;same&nbsp;as&nbsp;a&nbsp;capturing&nbsp;group;&nbsp;&nbsp;<BR>you&nbsp;can&nbsp;put&nbsp;anything&nbsp;inside&nbsp;it,&nbsp;repeat&nbsp;it&nbsp;with&nbsp;a&nbsp;repetition&nbsp;metacharacter&nbsp;&nbsp;<BR>such&nbsp;as&nbsp;&quot;*&quot;,&nbsp;and&nbsp;nest&nbsp;it&nbsp;within&nbsp;other&nbsp;groups&nbsp;(capturing&nbsp;or&nbsp;non-capturing).&nbsp;&nbsp;<BR>(?:...)&nbsp;is&nbsp;particularly&nbsp;useful&nbsp;when&nbsp;modifying&nbsp;an&nbsp;existing&nbsp;group,&nbsp;since&nbsp;you&nbsp;&nbsp;<BR>can&nbsp;add&nbsp;new&nbsp;groups&nbsp;without&nbsp;changing&nbsp;how&nbsp;all&nbsp;the&nbsp;other&nbsp;groups&nbsp;are&nbsp;numbered.&nbsp;&nbsp;<BR>It&nbsp;should&nbsp;be&nbsp;mentioned&nbsp;that&nbsp;there's&nbsp;no&nbsp;performance&nbsp;difference&nbsp;in&nbsp;searching&nbsp;&nbsp;<BR>between&nbsp;capturing&nbsp;and&nbsp;non-capturing&nbsp;groups;&nbsp;neither&nbsp;form&nbsp;is&nbsp;any&nbsp;faster&nbsp;than&nbsp;&nbsp;<BR>the&nbsp;other.&nbsp;&nbsp;<BR>&nbsp;<BR>The&nbsp;second,&nbsp;and&nbsp;more&nbsp;significant,&nbsp;feature,&nbsp;is&nbsp;named&nbsp;groups;&nbsp;instead&nbsp;of&nbsp;&nbsp;<BR>referring&nbsp;to&nbsp;them&nbsp;by&nbsp;numbers,&nbsp;groups&nbsp;can&nbsp;be&nbsp;referenced&nbsp;by&nbsp;a&nbsp;name.&nbsp;&nbsp;<BR>&nbsp;<BR>The&nbsp;syntax&nbsp;for&nbsp;a&nbsp;named&nbsp;group&nbsp;is&nbsp;one&nbsp;of&nbsp;the&nbsp;Python-specific&nbsp;extensions:&nbsp;&nbsp;<BR>(?P&lt;name&gt;...).&nbsp;name&nbsp;is,&nbsp;obviously,&nbsp;the&nbsp;name&nbsp;of&nbsp;the&nbsp;group.&nbsp;Except&nbsp;for&nbsp;&nbsp;<BR>associating&nbsp;a&nbsp;name&nbsp;with&nbsp;a&nbsp;group,&nbsp;named&nbsp;groups&nbsp;also&nbsp;behave&nbsp;identically&nbsp;to&nbsp;&nbsp;<BR>capturing&nbsp;groups.&nbsp;The&nbsp;MatchObject&nbsp;methods&nbsp;that&nbsp;deal&nbsp;with&nbsp;capturing&nbsp;groups&nbsp;&nbsp;<BR>all&nbsp;accept&nbsp;either&nbsp;integers,&nbsp;to&nbsp;refer&nbsp;to&nbsp;groups&nbsp;by&nbsp;number,&nbsp;or&nbsp;a&nbsp;string&nbsp;&nbsp;<BR>containing&nbsp;the&nbsp;group&nbsp;name.&nbsp;Named&nbsp;groups&nbsp;are&nbsp;still&nbsp;given&nbsp;numbers,&nbsp;so&nbsp;you&nbsp;&nbsp;<BR>can&nbsp;retrieve&nbsp;information&nbsp;about&nbsp;a&nbsp;group&nbsp;in&nbsp;two&nbsp;ways:&nbsp;&nbsp;<BR>&nbsp;<BR>＞<I>&gt;&gt;&nbsp;p&nbsp;=&nbsp;re.compile(r'(?P&lt;word&gt;\b\w+\b)')&nbsp;</I><BR>＞<I>&gt;&gt;&nbsp;m&nbsp;=&nbsp;p.search(&nbsp;'((((&nbsp;Lots&nbsp;of&nbsp;punctuation&nbsp;)))'&nbsp;)&nbsp;</I><BR>＞<I>&gt;&gt;&nbsp;m.group('word')&nbsp;</I><BR>'Lots'&nbsp;<BR>＞<I>&gt;&gt;&nbsp;m.group(1)&nbsp;</I><BR>'Lots'&nbsp;<BR>&nbsp;<BR>Named&nbsp;groups&nbsp;are&nbsp;handy&nbsp;because&nbsp;they&nbsp;let&nbsp;you&nbsp;use&nbsp;easily-remembered&nbsp;names,&nbsp;&nbsp;<BR>instead&nbsp;of&nbsp;having&nbsp;to&nbsp;remember&nbsp;numbers.&nbsp;Here's&nbsp;an&nbsp;example&nbsp;RE&nbsp;from&nbsp;the&nbsp;imaplib&nbsp;&nbsp;<BR>module:&nbsp;&nbsp;<BR>&nbsp;<BR>InternalDate&nbsp;=&nbsp;re.compile(r'INTERNALDATE&nbsp;&quot;'&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;r'(?P&lt;day&gt;[&nbsp;123][0-9])-(?P&lt;mon&gt;[A-Z][a-z][a-z])-'&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;r'(?P&lt;year&gt;[0-9][0-9][0-9][0-9])'&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;r'&nbsp;(?P&lt;hour&gt;[0-9][0-9]):(?P&lt;min&gt;[0-9][0-9]):(?P&lt;sec&gt;[0-9][0-9])'&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;r'&nbsp;(?P&lt;zonen&gt;[-+])(?P&lt;zoneh&gt;[0-9][0-9])(?P&lt;zonem&gt;[0-9][0-9])'&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;r'&quot;')&nbsp;<BR>&nbsp;<BR>It's&nbsp;obviously&nbsp;much&nbsp;easier&nbsp;to&nbsp;retrieve&nbsp;m.group('zonem'),&nbsp;instead&nbsp;of&nbsp;having&nbsp;&nbsp;<BR>to&nbsp;remember&nbsp;to&nbsp;retrieve&nbsp;group&nbsp;9.&nbsp;Since&nbsp;the&nbsp;syntax&nbsp;for&nbsp;backreferences&nbsp;refers&nbsp;&nbsp;<BR>to&nbsp;the&nbsp;number&nbsp;of&nbsp;the&nbsp;group,&nbsp;in&nbsp;an&nbsp;expression&nbsp;like&nbsp;(...)\1,&nbsp;there's&nbsp;naturally&nbsp;&nbsp;<BR>a&nbsp;variant&nbsp;that&nbsp;uses&nbsp;the&nbsp;group&nbsp;name&nbsp;instead&nbsp;of&nbsp;the&nbsp;number.&nbsp;This&nbsp;is&nbsp;also&nbsp;a&nbsp;&nbsp;<BR>Python&nbsp;extension:&nbsp;(?P=name)&nbsp;indicates&nbsp;that&nbsp;the&nbsp;contents&nbsp;of&nbsp;the&nbsp;group&nbsp;called&nbsp;&nbsp;<BR>name&nbsp;should&nbsp;again&nbsp;be&nbsp;found&nbsp;at&nbsp;the&nbsp;current&nbsp;point.&nbsp;The&nbsp;regular&nbsp;expression&nbsp;for&nbsp;&nbsp;<BR>finding&nbsp;doubled&nbsp;words,&nbsp;(\b\w+)\s+\1&nbsp;can&nbsp;also&nbsp;be&nbsp;written&nbsp;as&nbsp;&nbsp;<BR>(?P&lt;word&gt;\b\w+)\s+(?P=word):&nbsp;&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>＞<I>&gt;&gt;&nbsp;p&nbsp;=&nbsp;re.compile(r'(?P&lt;word&gt;\b\w+)\s+(?P=word)')&nbsp;</I><BR>＞<I>&gt;&gt;&nbsp;p.search('Paris&nbsp;in&nbsp;the&nbsp;the&nbsp;spring').group()&nbsp;</I><BR>'the&nbsp;the'&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>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -