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<HTML><HEAD><TITLE>Recipe 6.6. Matching Multiple Lines (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:33:48Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch06_01.htm"TITLE="6. Pattern Matching"><LINKREL="prev"HREF="ch06_06.htm"TITLE="6.5. Finding the Nth Occurrence of a Match"><LINKREL="next"HREF="ch06_08.htm"TITLE="6.7. Reading Records with a Pattern Separator"></HEAD><BODYBGCOLOR="#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="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch06_06.htm"TITLE="6.5. Finding the Nth Occurrence of a Match"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 6.5. Finding the Nth Occurrence of a Match"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch06_01.htm"TITLE="6. Pattern Matching"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch06_08.htm"TITLE="6.7. Reading Records with a Pattern Separator"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 6.7. Reading Records with a Pattern Separator"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch06-14503">6.6. Matching Multiple Lines</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch06-pgfId-755">Problem<ACLASS="indexterm"NAME="ch06-idx-1000007578-0"></A><ACLASS="indexterm"NAME="ch06-idx-1000007578-1"></A><ACLASS="indexterm"NAME="ch06-idx-1000007578-2"></A><ACLASS="indexterm"NAME="ch06-idx-1000007578-3"></A><ACLASS="indexterm"NAME="ch06-idx-1000007578-4"></A></A></H3><PCLASS="para">You want to use regular expressions on a string containing more than one line, but the special characters <CODECLASS="literal">.</CODE> (any character but newline), <CODECLASS="literal">^</CODE> (start of string), and <CODECLASS="literal">$</CODE> (end of string) don't seem to work for you. This might happen if you're reading in multiline records or the whole file at once.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch06-pgfId-761">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">/m</CODE><ACLASS="indexterm"NAME="ch06-idx-1000007582-0"></A><ACLASS="indexterm"NAME="ch06-idx-1000007582-1"></A>, <CODECLASS="literal">/s</CODE>, or both as pattern modifiers. <CODECLASS="literal">/s</CODE> lets <CODECLASS="literal">.</CODE> match newline (normally it doesn't). If the string had more than one line in it, then <CODECLASS="literal">/foo.*bar/s</CODE> could match a <CODECLASS="literal">"foo"</CODE> on one line and a <CODECLASS="literal">"bar"</CODE> on a following line. This doesn't affect dots in character classes like <CODECLASS="literal">[#%.]</CODE>, since they are regular periods anyway.</P><PCLASS="para">The <CODECLASS="literal">/m</CODE> modifier lets <CODECLASS="literal">^</CODE> and <CODECLASS="literal">$</CODE> match next to a newline. <CODECLASS="literal">/^=head[1-7]$/m</CODE> would match that pattern not just at the beginning of the record, but anywhere right after a newline as well.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch06-pgfId-769">Discussion</A></H3><PCLASS="para">A common, brute-force approach to parsing documents where newlines are not significant is to read the file one paragraph at a time (or sometimes even the entire file as one string) and then extract tokens one by one. To match across newlines, you need to make <CODECLASS="literal">.</CODE> match a newline; it ordinarily does not. In cases where newlines are important and you've read more than one line into a string, you'll probably prefer to have <CODECLASS="literal">^</CODE> and <CODECLASS="literal">$</CODE> match beginning- and end-of-line, not just beginning- and end-of-string.</P><PCLASS="para">The difference between <CODECLASS="literal">/m</CODE> and <CODECLASS="literal">/s</CODE> is important: <CODECLASS="literal">/m</CODE> makes <CODECLASS="literal">^</CODE> and <CODECLASS="literal">$</CODE> match next to a newline, while <CODECLASS="literal">/s</CODE> makes <CODECLASS="literal">.</CODE> match newlines. You can even use them together - they're not mutually exclusive options.</P><PCLASS="para"><ACLASS="xref"HREF="ch06_07.htm#ch06-10877"TITLE="killtags">Example 6.2</A> creates a filter to strip HTML tags out of each file in <CODECLASS="literal">@ARGV</CODE> and send the results to STDOUT. First we undefine the record separator so each read operation fetches one entire file. (There could be more than one file, because <CODECLASS="literal">@ARGV</CODE> has several arguments in it. In this case, each read would get a whole file.) Then we strip out instances of beginning and ending angle brackets, plus anything in between them. We can't use just <CODECLASS="literal">.*</CODE> for two reasons: first, it would match closing angle brackets, and second, the dot wouldn't cross newline boundaries. Using <CODECLASS="literal">.*?</CODE> in conjunction with <CODECLASS="literal">/s</CODE> solves these problems - at least in this case.</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch06-10877">Example 6.2: killtags</A></H4><PRECLASS="programlisting">#!/usr/bin/perl# <ACLASS="indexterm"NAME="ch06-idx-1000007791-0"></A>killtags - very bad html tag killerundef $/; # each read is whole filewhile (<>) { # get one whole file at a time s/<.*?>//gs; # strip tags (terribly) print; # print file to STDOUT}</PRE></DIV><PCLASS="para">Because this is just a single character, it would be much faster to use <CODECLASS="literal">s/<[^>]*>//gs,</CODE> but that's still a na飗e approach: It doesn't correctly handle tags inside HTML comments or angle brackets in quotes (<<CODECLASS="literal">IMG</CODE> <CODECLASS="literal">SRC="here.gif"</CODE> <CODECLASS="literal">ALT="<<Ooh</CODE> <CODECLASS="literal">la</CODE> <CODECLASS="literal">la!>>"></CODE>). <ACLASS="xref"HREF="ch20_07.htm"TITLE="Extracting or Removing HTML Tags">Recipe 20.6</A> explains how to avoid these problems.</P><PCLASS="para"><ACLASS="xref"HREF="ch06_07.htm#ch06-31611"TITLE="headerfy">Example 6.3</A> takes a plain text document and looks for lines at the start of paragraphs that look like <CODECLASS="literal">"Chapter</CODE> <CODECLASS="literal">20:</CODE> <CODECLASS="literal">Better</CODE> <CODECLASS="literal">Living</CODE> <CODECLASS="literal">Through</CODE> <CODECLASS="literal"⌨️ 快捷键说明
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