More Record Oriented Reading

by Andrew Johnson

May 16, 2001 —

Last week, we looked at three common cases for defining the 'record'
that Perl's input operator reads: line oriented records, paragraph
oriented records, and file oriented records. Now we'll look at grabbing
and parsing records in a more custom format.

One common type of multi-line record is the name/value pair format:

name = andrew
age = 37
beer = dark ale
*****
name = john
beer = pale ale
age = 35
*****

In this case, we have records consisting of three named fields
separated by "*****\n". We can simply read this data in and output it
with a different record separator:

$/ = "*****\n";
while(<>){
chomp;
print "$_#####\n";
}

Not very exciting, but it illustrates the concept of the input record
separator. By setting the input record separator to the
string "*****\n", when we read from the file the input operator reads
one multi-line record at a time: everything up to and including the
next record separator string is returned. This also illustrates that
chomp() removes any trailing record separator, not just any trailing
newline character (and not just the last character of the string as
does the chop() function). In the above, we are merely removing the
record separator and producing output with a new separator string.

A slightly less trivial task might be to convert such a file into a csv
style data file. Let's assume we've been asked to read a file such as
the above and produce a csv file with a ':' as the field separator and
ordered fields: name, age, beer.

$/ = "*****\n";
my @fields = qw/name age beer/;
while(){
chomp;
my %hash = split /\s*[\n=]\s*/;
print join(':',@hash{@fields}),"\n";
}

__DATA__
name = andrew
age = 37
beer = dark ale
*****
name = john
beer = pale ale
age = 35
*****

Here we've chomp()'d off the record separator and created a hash by
splitting the record at every '=' and "\n" character. We were careful
to account for possible whitespace around the '=' character in our split
(), but we may need to be a little more careful: What if one of the
field lines is indented? In that case we would have a hash key like "
name" instead of just "name", and our hash slice to produce the output
wouldn't work as we wished. A more careful approach would be to split
the data into an array and strip any leading spaces before we create
our hash:

$/ = "*****\n";
my @fields = qw/name age beer/;
while(){
chomp;
my @data = split /\s*[\n=]\s*/;
s/^\s+// for @data;

my %hash = @data;
print join(':',@hash{@fields}),"\n";
}

__DATA__
name = andrew
age = 37
beer = dark ale
*****
name = john
beer = pale ale
age = 35
*****

Whatever the form of your plain text database, be it csv or multi-line
records, Perl makes it rather easy to process the data one record at a
time.

Next Week: Special Perl Variables.

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