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8.1. Reading Lines with Continuation Characters

Problem

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.

Solution

Build up the complete lines one at a time until reaching one without a backslash:

while (defined($line = <FH>) ) {
    chomp $line;
    if ($line =~ s/\\$//) {
        $line .= <FH>;
        redo unless eof(FH);
    }
    # process full record in $line here
}

Discussion

Here's an example input file:

DISTFILES = $(DIST_COMMON) $(SOURCES) $(HEADERS) \
        $(TEXINFOS) $(INFOS) $(MANS) $(DATA)
DEP_DISTFILES = $(DIST_COMMON) $(SOURCES) $(HEADERS) \
        $(TEXINFOS) $(INFO_DEPS) $(MANS) $(DATA) \
        $(EXTRA_DIST)

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.

Here's how the algorithm works. The while loop reads lines, which may or may not be complete records  - they might end in backslash (and a newline). The substitution operator s/// 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 $line variable, and use redo to jump back to just inside the opening brace of the while loop. This lands us back on the chomp.

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:

if ($line =~ s/\\\s*$//) { 
    # as before
}

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.

See Also

The chomp function in perlfunc (1) and in Chapter 3 of Programming Perl; the redo keyword in the "Loop Control" sections of perlsyn (1) and Chapter 2 of Programming Perl


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