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<html><head><title>The Match Variables (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="ch09_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="ch09_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">9.5. The Match Variables</h2><p><a name="INDEX-595" /> <a name="INDEX-596" />Doyou remember the regular expression memories, which we used withbackreferences in the previous chapter? Those memories are alsoavailable after the pattern match is done, after we return to Perl.They're strings, so they are kept in scalar variables withnames like <tt class="literal">$1</tt> and <tt class="literal">$2</tt>. There areas many of these variables as there are pairs of memory parenthesesin the pattern. As you'd expect, <tt class="literal">$4</tt> meansthe string matched by the fourth set of parentheses. This is the samestring that <tt class="literal">\4</tt> referred to inside the patternmatch.</p><p>Why are there two different ways to refer to that same string?They're not really referring to the same string at the sametime; <tt class="literal">$4</tt> means the fourth memory of an<em class="emphasis">already</em> <em class="emphasis">completed</em> patternmatch, while <tt class="literal">\4</tt> is a backreference referring backto the fourth memory of the <em class="emphasis">currently</em><em class="emphasis">matching</em> regular expression. Besides,backreferences work inside regular expressions only; once we'reback in the world of Perl, we'll use <tt class="literal">$4</tt>.</p><p>These match variables are a big part of the power of regularexpressions, because they let us pull out the parts of a string:</p><blockquote><pre class="code">$_ = "Hello there, neighbor";if (/\s(\w+),/) { # memorize the word between space and comma print "the word was $1\n"; # the word was there}</pre></blockquote><p>Or you could use more than one memory at once:</p><blockquote><pre class="code">$_ = "Hello there, neighbor";if (/(\S+) (\S+), (\S+)/) { print "words were $1 $2 $3\n";}</pre></blockquote><p>That tells us that the <tt class="literal">words were Hello thereneighbor</tt>. Notice that there's no comma in the output(because the comma is outside of the memory parentheses). That leavesthe comma out of memory two. Using this technique, we can chooseexactly what we want in the memories, as well as what we want toleave out.</p><p>You could even have an empty match variable,<a href="#FOOTNOTE-199">[199]</a> if that part of thepattern might be empty. That is, a match variable may contain theempty string:</p><blockquote class="footnote"> <a name="FOOTNOTE-199" /><p>[199]Asopposed to an undefined one. If you have three or fewer sets ofparentheses in the pattern, <tt class="literal">$4</tt> will be<tt class="literal">undef</tt>.</p> </blockquote><blockquote><pre class="code">my $dino = "I fear that I'll be extinct after 1000 years.";if ($dino =~ /(\d*) years/) { print "That said '$1' years.\n"; # 1000}$dino = "I fear that I'll be extinct after a few million years.";if ($dino =~ /(\d*) years/) { print "That said '$1' years.\n"; # empty string}</pre></blockquote><a name="lperl3-CHP-9-SECT-5.1" /><div class="sect2"><h3 class="sect2">9.5.1. The Persistence of Memory</h3><p>These match variables generally stay around until the next<em class="emphasis">successful</em> pattern match.<a href="#FOOTNOTE-200">[200]</a> That is, anunsuccessful match leaves the previous <a name="INDEX-597" />memories intact, but a successfulone resets them all. But this correctly implies that youshouldn't use these match variables unless the match succeeded;otherwise, you could be seeing a memory from some previous pattern.The following (bad) example is supposed to print a word matched from<tt class="literal">$_</tt>. But if the match fails, it's usingwhatever leftover string happens to be found in<tt class="literal">$1</tt>:</p><blockquote class="footnote"> <a name="FOOTNOTE-200" /><p>[200]Theactual scoping rule is much more complex (see the documentation ifyou need it), but as long as you don't expect the matchvariables to be untouched many lines after a pattern match, youshouldn't have problems.</p> </blockquote><blockquote><pre class="code">$wilma =~ /(\w+)/; # BAD! Untested match resultprint "Wilma's word was $1... or was it?\n";</pre></blockquote><p>This is another reason that a pattern match is almost always found inthe conditional expression of an <tt class="literal">if</tt> or<tt class="literal">while</tt>:</p><blockquote><pre class="code">if ($wilma =~ /(\w+)/) { print "Wilma's word was $1.\n";} else { print "Wilma doesn't have a word.\n";}</pre></blockquote><p>Since these memories don't stay around forever, youshouldn't use a match variable like <tt class="literal">$1</tt> morethan a few lines after its pattern match. If your maintenanceprogrammer adds a new regular expression between your regularexpression and your use of <tt class="literal">$1</tt>, you'll begetting the value of <tt class="literal">$1</tt> for the second match,rather than the first. For this reason, if you need a memory for morethan a few lines, it's generally best to copy it into anordinary variable. Doing this helps make the code more readable atthe same time:</p><blockquote><pre class="code">if ($wilma =~ /(\w+)/) { my $wilma_word = $1; ...}</pre></blockquote><p>Later, in <a href="ch14_01.htm">Chapter 14, "Process Management"</a>, we'll see how to getthe memory value <em class="emphasis">directly</em> into the variable atthe same time as the pattern match happens, without having to use<tt class="literal">$1</tt> explicitly.</p></div><a name="lperl3-CHP-9-SECT-5.2" /><div class="sect2"><h3 class="sect2">9.5.2. The Automatic Match Variables</h3><p>There are three more match variables that you get for free,<a href="#FOOTNOTE-201">[201]</a> whetherthe pattern has memory parentheses or not. That's the goodnews; the bad news is that these variables have weird names.</p><blockquote class="footnote"><a name="FOOTNOTE-201" /><p>[201]Yeah, right. There's no such thing as a free match. Theseare "free" only in the sense that they don'trequire match parentheses. Don't worry; we'll mentiontheir real cost a little later, though.</p> </blockquote><p>Now, Larry probably would have been happy enough to call these byslightly-less-weird names, like perhaps <tt class="literal">$gazoo</tt> or<tt class="literal">$ozmodiar</tt>. But those are names that you just mightwant to use in your own code. To keep ordinary Perl programmers fromhaving to memorize the names of <em class="emphasis">all</em> ofPerl's special variables before choosing their first variablenames in their first programs,<a href="#FOOTNOTE-202">[202]</a> Larry has given strange names to many of Perl's<a name="INDEX-598" /><a name="INDEX-599" /><a name="INDEX-600" />builtin variables, names that"break the rules." In this case, the names arepunctuation marks: <tt class="literal">$&</tt>, <tt class="literal">$`</tt>,and <tt class="literal">$'</tt>. They're strange, ugly, and weird,but those are their names.<a href="#FOOTNOTE-203">[203]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-202" /><p>[202]You should still avoida few classical variable names like <tt class="literal">$ARGV</tt>, butthese few are all in all-caps. All of Perl's builtin variablesare documented in the <em class="emphasis">perlvar</em> manpage.</p></blockquote><blockquote class="footnote"> <a name="FOOTNOTE-203" /><p>[203]If you really can'tstand these names, check out the <tt class="literal">English</tt> module,which attempts to give all of Perl's strangest variables nearlynormal names. But the use of this module has never really caught on;instead, Perl programmers have grown to love the punctuation-markvariable names, strange as they are. </p> </blockquote><p>The part of the string that actually matched the pattern isautomatically stored in<tt class="literal">$&</tt><a name="INDEX-601" />:</p><blockquote><pre class="code">if ("Hello there, neighbor" =~ /\s(\w+),/) { print "That actually matched '$&'.\n";}</pre></blockquote><p>That tells us that the part that matched was <tt class="literal">"there,"</tt> (with a space, a word, and a comma). Memory one, in<tt class="literal">$1</tt>, has just the five-letter word<tt class="literal">there</tt>, but <tt class="literal">$&</tt> has theentire matched section.</p><p>Whatever came before the matched section is in<tt class="literal">$`</tt><a name="INDEX-602" />, and whatever was after it is in<tt class="literal">$'</tt><a name="INDEX-603" />. Another way to say that is that<tt class="literal">$`</tt> holds whatever the regular expression enginehad to skip over before it found the match, and <tt class="literal">$'</tt>has the remainder of the string that the pattern never got to. If youglue these three strings together in order, you'll always getback the original string:</p><blockquote><pre class="code">if ("Hello there, neighbor" =~ /\s(\w+),/) { print "That was ($`)($&)($').\n";}</pre></blockquote><p>The message shows the string as <tt class="literal">(Hello)( there,)(neighbor)</tt>, showing the three automatic match variables inaction. This may seem familiar, and for good reason: These automaticmemory variables are what the pattern test program (from <a href="ch07_01.htm">Chapter 7, "Concepts of Regular Expressions"</a>) was using in its line of"mystery" code, to show what part of the string was beingmatched by the pattern:</p><blockquote><pre class="code">print "Matched: |<em class="emphasis">$`</em><<em class="emphasis">$&</em>><em class="emphasis">$'</em>|\n"; # The three automatic match variables</pre></blockquote><p>Any or all of these three automatic match variables may be empty, ofcourse, just like the numbered match variables. And they have thesame scope as the numbered match variables. Generally, that meansthat they'll stay around until the next successful patternmatch.</p><p>Now, we said earlier that these three are "free." Well,freedom has its price. In this case, the price is that once you useany one of these automatic match variables anywhere in your entireprogram, other regular expressions will run a little more slowly.Now, this isn't a giant slowdown, but it's enough of aworry that many Perl programmers will simply never use these<a name="INDEX-604" /><a name="INDEX-605" />automatic matchvariables.<a href="#FOOTNOTE-204">[204]</a> Instead, they'll usea workaround. For example, if the only one you need is<tt class="literal">$&</tt>, just put parentheses around the wholepattern and use <tt class="literal">$1</tt> instead (you may need torenumber the pattern's memories, of course).</p><blockquote class="footnote"> <a name="FOOTNOTE-204" /><p>[204]Most of these folks haven't actuallybenchmarked their programs to see whether their workarounds actuallysave time, though; it's as though these variables werepoisonous or something. But we can't blame them for notbenchmarking -- many programs that could benefit from these threevariables take up only a few minutes of CPU time in a week, sobenchmarking and optimizing would be a waste of time. But in thatcase, why fear a possible extra millisecond? By the way, the Perldevelopers are working on this problem, but there will probably be nosolution before Perl 6.</p> </blockquote><p>Match variables (both the automatic ones and the numbered ones) aremost often used in substitutions, which are the topic of the nextsection.<a name="INDEX-606" /><a name="INDEX-607" /></p></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch09_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="ch09_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">9.4. Interpolating into Patterns</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">9.6. Substitutions with s///</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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