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<HTML><HEAD><TITLE>Recipe 8.9. Processing Variable-Length Text Fields (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="1999-07-02T01:38:45Z"><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="ch08_01.htm"TITLE="8. File Contents"><LINKREL="prev"HREF="ch08_09.htm"TITLE="8.8. Reading a Particular Line in a File"><LINKREL="next"HREF="ch08_11.htm"TITLE="8.10. Removing the Last Line of a File"></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="ch08_09.htm"TITLE="8.8. Reading a Particular Line in a File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.8. Reading a Particular Line in a File"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch08_01.htm"TITLE="8. File Contents"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch08_11.htm"TITLE="8.10. Removing the Last Line of a File"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 8.10. Removing the Last Line of a File"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch08-chap08_processing_1">8.9. Processing Variable-Length Text Fields</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-954">Problem<ACLASS="indexterm"NAME="ch08-idx-1000004670-0"></A><ACLASS="indexterm"NAME="ch08-idx-1000004670-1"></A><ACLASS="indexterm"NAME="ch08-idx-1000004670-2"></A><ACLASS="indexterm"NAME="ch08-idx-1000004670-3"></A><ACLASS="indexterm"NAME="ch08-idx-1000004670-4"></A></A></H3><PCLASS="para">You want to extract variable length fields from your input.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-960">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">split</CODE> with a pattern matching the field separators.</P><PRECLASS="programlisting"># given $RECORD with field separated by PATTERN,# extract @FIELDS.@FIELDS = split(/PATTERN/, $RECORD);</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-972">Discussion</A></H3><PCLASS="para">The <CODECLASS="literal">split</CODE> function takes up to three arguments: <CODECLASS="literal">PATTERN</CODE>, <CODECLASS="literal">EXPRESSION</CODE>, and <CODECLASS="literal">LIMIT</CODE>. The <CODECLASS="literal">LIMIT</CODE> parameter is the maximum number of fields to split into. (If the input contains more fields, they are returned unsplit in the final list element.) If <CODECLASS="literal">LIMIT</CODE> is omitted, all fields (except any final empty ones) are returned. <CODECLASS="literal">EXPRESSION</CODE> gives the string value to split. If <CODECLASS="literal">EXPRESSION</CODE> is omitted, <CODECLASS="literal">$_</CODE> is split. <CODECLASS="literal">PATTERN</CODE> is a pattern matching the field separator. If <CODECLASS="literal">PATTERN</CODE> is omitted, contiguous stretches of whitespace are used as the field separator and leading empty fields are silently discarded.</P><PCLASS="para">If your input field separator isn't a fixed string, you might want <CODECLASS="literal">split</CODE> to return the field separators as well as the data by using parentheses in <CODECLASS="literal">PATTERN</CODE> to save the field separators. For instance:</P><PRECLASS="programlisting">split(/([+-])/, "3+5-2");</PRE><PCLASS="para">returns the values:</P><PRECLASS="programlisting">(3, '+', 5, '-', 2)</PRE><PCLASS="para">To split colon-separated records in the style of the <EMCLASS="emphasis">/etc/passwd</EM> file, use:</P><PRECLASS="programlisting">@fields = split(/:/, $RECORD);</PRE><PCLASS="para">The classic application of <CODECLASS="literal">split</CODE> is whitespace-separated records:</P><PRECLASS="programlisting">@fields = split(/\s+/, $RECORD);</PRE><PCLASS="para">If <CODECLASS="literal">$RECORD</CODE> started with whitespace, this last use of <CODECLASS="literal">split</CODE> would have put an empty string into the first element of <CODECLASS="literal">@fields</CODE> because <CODECLASS="literal">split</CODE> would consider the record to have an initial empty field. If you didn't want this, you could use this special form of <CODECLASS="literal">split</CODE>:</P><PRECLASS="programlisting">@fields = split(" ", $RECORD);</PRE><PCLASS="para">This behaves like <CODECLASS="literal">split</CODE> with a pattern of <CODECLASS="literal">/\s+/</CODE>, but ignores leading whitespace.</P><PCLASS="para">When the record separator can appear in the record, you have a problem. The usual solution is to escape occurrences of the record separator in records by prefixing them with a backslash. See <ACLASS="xref"HREF="ch01_14.htm"TITLE="Escaping Characters">Recipe 1.13</A>.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1002">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">split</CODE> function in <ICLASS="filename">perlfunc </I>(1) and in <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch08_09.htm"TITLE="8.8. Reading a Particular Line in a File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.8. 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Removing the Last Line of a File</TD></TR></TABLE><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><FONTSIZE="-1"></DIV<!-- LIBRARY NAV BAR --> <img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p> <a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font> </p> <map name="library-map"> <area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map> </BODY></HTML>⌨️ 快捷键说明
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