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<HTML><HEAD><TITLE>Recipe 8.1. Reading Lines with Continuation Characters (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="1999-07-02T01:38:23Z"><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_01.htm"TITLE="8.0. Introduction"><LINKREL="next"HREF="ch08_03.htm"TITLE="8.2. Counting Lines (or Paragraphs or Records) in 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_01.htm"TITLE="8.0. Introduction"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.0. Introduction"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_03.htm"TITLE="8.2. Counting Lines (or Paragraphs or Records) in a File"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 8.2. Counting Lines (or Paragraphs or Records) in a File"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch08-chap08_reading_0">8.1. Reading Lines with Continuation Characters</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-180">Problem<ACLASS="indexterm"NAME="ch08-idx-1000004612-0"></A><ACLASS="indexterm"NAME="ch08-idx-1000004612-1"></A><ACLASS="indexterm"NAME="ch08-idx-1000004612-2"></A></A></H3><PCLASS="para">You have a file with long lines split over two or more lines, with backslashes to indicate that a continuation line follows. You want to rejoin those split lines. Makefiles, shell scripts, and many other scripting or configuration languages let you break a long line into several shorter ones in this fashion.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-186">Solution</A></H3><PCLASS="para">Build up the complete lines one at a time until reaching one without a backslash:</P><PRECLASS="programlisting">while (defined($line = <FH>) ) { chomp $line; if ($line =~ s/\\$//) { $line .= <FH>; redo unless eof(FH); } # process full record in $line here}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-208">Discussion</A></H3><PCLASS="para">Here's an example input file:</P><PRECLASS="programlisting">DISTFILES = $(DIST_COMMON) $(SOURCES) $(HEADERS) \ $(TEXINFOS) $(INFOS) $(MANS) $(DATA)DEP_DISTFILES = $(DIST_COMMON) $(SOURCES) $(HEADERS) \ $(TEXINFOS) $(INFO_DEPS) $(MANS) $(DATA) \ $(EXTRA_DIST)</PRE><PCLASS="para">You'd like to process that file with the escaped newlines ignored. That way the first record would in this case be the first two lines, the second record the next three lines, etc.</P><PCLASS="para">Here's how the algorithm works. The <CODECLASS="literal">while</CODE> loop reads lines, which may or may not be complete records - they might end in backslash (and a newline). The substitution operator <CODECLASS="literal">s///</CODE> tries to remove a trailing backslash. If the substitution fails, we've found a line without a backslash at the end. Otherwise, read another record, concatenate it onto the accumulating <CODECLASS="literal">$line</CODE> variable, and use <CODECLASS="literal">redo</CODE> to jump back to just inside the opening brace of the <CODECLASS="literal">while</CODE> loop. This lands us back on the <CODECLASS="literal">chomp</CODE>.</P><PCLASS="para">A common problem with files in this format is invisible blanks between the backslash and end of line. It would be more forgiving if the substitute were like this:</P><PRECLASS="programlisting">if ($line =~ s/\\\s*$//) { # as before}</PRE><PCLASS="para">Unfortunately, even if your program is forgiving, others doubtlessly aren't. Just remember to be liberal in what you accept and conservative in what you produce. <ACLASS="indexterm"NAME="ch08-idx-1000004614-0"></A><ACLASS="indexterm"NAME="ch08-idx-1000004614-1"></A><ACLASS="indexterm"NAME="ch08-idx-1000004614-2"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-238">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">chomp</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>; the redo keyword in the "Loop Control" sections of <ICLASS="filename">perlsyn</I> (1) and <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</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_01.htm"TITLE="8.0. Introduction"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.0. Introduction"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="book"HREF="index.htm"TITLE="Perl Cookbook"><IMGSRC="../gifs/txthome.gif"ALT="Perl Cookbook"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch08_03.htm"TITLE="8.2. Counting Lines (or Paragraphs or Records) in a File"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 8.2. Counting Lines (or Paragraphs or Records) in a File"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">8.0. Introduction</TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="index"HREF="index/index.htm"TITLE="Book Index"><IMGSRC="../gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228">8.2. 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