ch17_05.htm
来自「by Randal L. Schwartz and Tom Phoenix I」· HTM 代码 · 共 234 行
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<html><head><title>More Powerful Regular Expressions (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch17_04.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="ch17_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">17.5. More Powerful Regular Expressions</h2><p><a name="INDEX-1112" />After already reading three chaptersabout regular expressions, you know that they're a powerfulfeature in the core of Perl. But there are even more features thatthe Perl developers have added; we'll see some of the mostimportant ones in this section. At the same time, you'll see alittle more about the internal operation of the<a name="INDEX-1113" />regular expression engine.</p><a name="lperl3-CHP-17-SECT-5.1" /><div class="sect2"><h3 class="sect2">17.5.1. Non-greedy Quantifiers</h3><p><a name="INDEX-1114" />Thefour quantifiers we've already seen (in <a href="ch08_01.htm">Chapter 8, "More About Regular Expressions"</a>) are all <em class="firstterm">greedy</em>. Thatmeans that they match as much as they can, only to reluctantly givesome back if that's necessary to allow the overall pattern tosucceed. Here's an example: Suppose you're using thepattern <tt class="literal">/fred.+barney/</tt> on the string<tt class="literal">fred</tt> <tt class="literal">and barney went bowling lastnight</tt>. Of course, we know that the regular expression willmatch that string, but let's see how it goes aboutit.<a href="#FOOTNOTE-364">[364]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-364" /><p>[364]The regular expression engine makes a fewoptimizations that make the true story different than we tell ithere, and those optimizations change from one release of Perl to thenext. You shouldn't be able to tell from the functionality thatit's not doing as we say, though. If you want to know how itreally works, you should read the latest source code. Be sure tosubmit patches for any bugs you find.</p> </blockquote><p>First, of course, the subpattern <tt class="literal">fred</tt> matches theidentical literal string. The next part of the pattern is the<tt class="literal">.+</tt>, which matches any character except newline, atleast one time. But the plus quantifier is greedy; it prefers tomatch as much as possible. So it immediately matches all of the restof the string, including the word <tt class="literal">night</tt>. (This maysurprise you, but the story isn't over yet.)</p><p>Now the subpattern <tt class="literal">barney</tt> would like to match, butit can't -- we're at the end of the string. But sincethe <tt class="literal">.+</tt> could still be successful even if itmatched one fewer character, it reluctantly gives back the letter<tt class="literal">t</tt> at the end of the string. (It's greedy,but it wants the whole pattern to succeed even more than it wants tomatch everything all by itself.)</p><p>The subpattern <tt class="literal">barney</tt> tries again to match, andstill can't. So the <tt class="literal">.+</tt> gives back the letter<tt class="literal">h</tt> and lets it try again. One character afteranother, the <tt class="literal">.+</tt> gives back what it matched untilfinally it gives up all of the letters of <tt class="literal">barney</tt>.Now, finally, the subpattern <tt class="literal">barney</tt> can match, andthe overall match succeeds.</p><p>Regular expression engines do a lot of backtracking like that, tryingevery different way of fitting the pattern to the string until one ofthem succeeds, or until none of them has.<a href="#FOOTNOTE-365">[365]</a> But as you could see fromthis example, that can involve a lot of backtracking, as thequantifier gobbles up too much of the string and has to be forced toreturn some of it.</p><blockquote class="footnote"> <a name="FOOTNOTE-365" /><p>[365]In fact,some regular expression engines try every different way, evencontinuing on <em class="emphasis">after</em> they find one that fits. ButPerl's regular expression engine is primarily interested inwhether the pattern can or cannot match, so finding even one matchmeans that the engine's work is done. Again, see JeffreyFriedl's <a name="INDEX-1115" /><em class="emphasis">Mastering RegularExpressions</em>.</p> </blockquote><p>For each of the greedy quantifiers, though, there's also anon-greedy quantifier available. Instead of the plus(<tt class="literal">+</tt>), we can use the non-greedy quantifier<tt class="literal">+?</tt>, which matches one or more times (just as theplus does), except that it prefers to match as few times as possible,rather than as many as possible. Let's see how that newquantifier works when the pattern is rewritten as<tt class="literal">/fred.+?barney/</tt>.</p><p>Once again, <tt class="literal">fred</tt> matches right at the start. Butthis time the next part of the pattern is <tt class="literal">.+?</tt>,which would prefer to match no more than one character, so it matchesjust the space after <tt class="literal">fred</tt>. The next subpattern is<tt class="literal">barney</tt>, but that can't match here (since thestring at the current position begins with <tt class="literal">andbarney</tt>...). So the <tt class="literal">.+?</tt> reluctantlymatches the <tt class="literal">a</tt> and lets the rest of the pattern tryagain. Once again, <tt class="literal">barney</tt> can't match, sothe <tt class="literal">.+?</tt> accepts the letter <tt class="literal">n</tt>and so on. Once the <tt class="literal">.+?</tt> has matched fivecharacters, <tt class="literal">barney</tt> can match, and the pattern is asuccess.</p><p>There was still some backtracking, but since the engine had to goback and try again just a few times, it should be a big improvementin speed. Well, it's an improvement if you'll generallyfind <tt class="literal">barney</tt> near <tt class="literal">fred</tt>. If yourdata often had <tt class="literal">fred</tt> near the start of the stringand <tt class="literal">barney</tt> only at the end, the greedy quantifiermight be a faster choice. In the end, the speed of the regularexpression depends upon the data.</p><p>But the non-greedy quantifiers aren't just about efficiency.Although they'll always match (or fail to match) the samestrings as their greedy counterparts, they may match differentamounts of the strings. For example, suppose you had someHTML-like<a href="#FOOTNOTE-366">[366]</a> text, and you want to remove all ofthe tags <tt class="literal"><BOLD></tt> and<tt class="literal"></BOLD></tt>, leaving their contents intact.Here's the text:</p><blockquote class="footnote"> <a name="FOOTNOTE-366" /><p>[366]Once again, we aren't using real HTMLbecause you can't correctly parse HTML with simple regularexpressions. If you really need to work with HTML or a similar markuplanguage, use a module that's made to handle thecomplexities.</p> </blockquote><blockquote><pre class="code">I'm talking about the cartoon with Fred and <BOLD>Wilma</BOLD>!</pre></blockquote><p>And here's a substitution to remove those tags. Butwhat's wrong with it?</p><blockquote><pre class="code">s#<BOLD>(.*)</BOLD>#$1#g;</pre></blockquote><p>The problem is that the star is greedy.<a href="#FOOTNOTE-367">[367]</a> What if the text had said this instead?</p><blockquote class="footnote"> <a name="FOOTNOTE-367" /><p>[367]There'sanother possible problem: we should have used the <tt class="literal">/s</tt>modifier as well, since the end tag may be on a differentline than the start tag. It's a good thing that this is just anexample; if we were writing something like this for real, we wouldhave taken our own advice and used a well-written module.</p></blockquote><blockquote><pre class="code">I thought you said Fred and <BOLD>Velma</BOLD>, not <BOLD>Wilma</BOLD></pre></blockquote><p>In that case, the pattern would match from the first<tt class="literal"><BOLD></tt> to the last<tt class="literal"></BOLD></tt>, leaving intact the ones in themiddle of the line. Oops! Instead, we want a non-greedy quantifier.The non-greedy form of star is <tt class="literal">*?</tt>, so thesubstitution now looks like this:</p><blockquote><pre class="code">s#<BOLD>(.*?)</BOLD>#$1#g;</pre></blockquote><p>And it does the right thing. </p><p>Since the non-greedy form of the plus was <tt class="literal">+?</tt> andthe non-greedy form of the star was <tt class="literal">*?</tt>,you've probably realized that the other two quantifiers looksimilar. The non-greedy form of any curly-brace quantifier looks thesame, but with a question mark after the closing brace, like<tt class="literal">{5,10}?</tt> or <tt class="literal">{8,}?</tt>.<a href="#FOOTNOTE-368">[368]</a> Andeven the question-mark quantifier has a non-greedy form:<tt class="literal">??</tt>. That matches either once or not at all, but itprefers not to match anything.<a name="INDEX-1116" /> </p><blockquote class="footnote"><a name="FOOTNOTE-368" /><p>[368]In theory, there's also a non-greedy quantifier form thatspecifies an exact number, like <tt class="literal">{3}?</tt>. But sincethat says to match exactly three of the preceding item, it has noflexibility to be either greedy or non-greedy.</p> </blockquote></div><a name="lperl3-CHP-17-SECT-5.2" /><div class="sect2"><h3 class="sect2">17.5.2. Matching Multiple-line Text</h3><p>Classic regular expressions were used to match just single lines of<a name="INDEX-1117" />text. But since Perl can work with stringsof any length, Perl's patterns can match multiple lines of textas easily as single lines. Of course, you have to include anexpression that holds more than one line of text. Here's astring that's four lines long:</p><blockquote><pre class="code">$_ = "I'm much better\nthan Barney is\nat bowling,\nWilma.\n";</pre></blockquote><p>Now, the<a name="INDEX-1118" />anchors <tt class="literal">^</tt> and<tt class="literal">$</tt> are normally anchors for the start and end ofthe whole string (see <a href="ch08_03.htm#lperl3-CHP-8-SECT-3">Section 8.3, "Anchors"</a> in<a href="ch08_01.htm">Chapter 8, "More About Regular Expressions"</a>). But the<tt class="literal">/m</tt><a name="INDEX-1119" /> regular expression option lets themmatch at internal newlines as well (think <tt class="literal">m</tt> formultiple lines). This makes them anchors for the start and end ofeach <em class="emphasis">line</em>, rather than the whole string. So thispattern can match:</p><blockquote><pre class="code">print "Found 'wilma' at start of line\n" if /^wilma\b/im;</pre></blockquote><p>Similarly, you could do a substitution on each line in a multilinestring. Here, we read an entire file into one variable,<a href="#FOOTNOTE-369">[369]</a> then add the file's name as aprefix at the start of each line:</p><blockquote class="footnote"><a name="FOOTNOTE-369" /><p>[369]Hope it's a small one. The file, that is, not thevariable.</p> </blockquote><a name="INDEX-1120" /><blockquote><pre class="code">open FILE, $filename or die "Can't open '$filename': $!";my $lines = join '', <FILE>;$lines =~ s/^/$filename: /gm;</pre></blockquote></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch17_04.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch17_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">17.4. Unquoted Hash Keys</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">17.6. 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