Parsing fixed-format lines


Some notes about parsing fixed-format lines in Perl.

In last post Resuming Data::Tubes I discussed a bit about a pipeline to parse a file that had dispositions of data, each composed of a sequence of fixed-format lines (called records).

For these lines, there’s a document (sorry… Italian only!) explaining the meaning of each line type, but the gist is that:

  • only a bunch of characters are allowed, encoded either in ASCII or in EBCDIC (this document’s appendix A has it all).
  • each line is always exactly 120 bytes/characters long, whose character index starts at 1.
  • each line has its own type that always appears in the same position in the field, i.e. positions 2 to 3 (two characters).
  • everything that follows is record/line type specific.

Some aggregates of records are grouped together depending on what is being transferred. As an example, a specific type of disposition is composed of 7 records, collectively holding all the info that is needed for the disposition.

In this case, the lines share some additional partial information - i.e. a “disposition local identifier” from character 4 to character 10 - then they restart holding type-specific data.

To address the parsing of these lines, I found it useful to define the mapping of the structure of each record type to the position of the sub-fields I was interested into:

state $parser_for = {
   IM => {
      data => 's 14-19',
   10 => {
      progressivo => 'n 4-10',
      'data-creazione' => 's 11-16',
      'data-valuta' => 's 17-22',
      causale => 'n 29-33',
      'importo-centesimi' => 'n 34-46',
      segno   => 's 47',
      riferimento => 's 58-69',
      'conto-creditore' => 'n 80-91',
   14 => {
      progressivo => 'n 4-10',
      data => 's 23-28',
      importo => 'n 34-46',
      segno => 'n 47',
      cliente => 's 98-113',

My parsing instructions are simple: n means numeric (where I remove leading 0 characters) and s means string (where I trim trailing spaces). A single integer means a single character, otherwise it’s ranges (usually one, possibly many). Hence, after detecting the record type, I do the parsing like this:

while (my ($key, $value) = each $parser_for->{$type}->%*) {
   my ($ft, @ranges) = split m{\s+}mxs, $value;
   $parsed{$key} = '';
   for my $range (@ranges) {
      my ($start, $stop) = split m{-}mxs, $range;
      $stop //= $start;
      $parsed{$key} .= substr $line, $start, $stop - $start;
   $parsed{$key} =~ s{\A0+}{}mxs if $ft eq 'n';
   $parsed{$key} =~ s{\s+\z}{}mxs if $ft eq 's';

The $start variable is decreased by one to account for the fact that strings are indexed starting at 0 in Perl. Variable $stop does not need this adjustment because it is supposed to point to the position immediately after the last character to extract with substr, which it already does!

So, here it is… my dumb-simple approach to parsing fixed-format lines. What would be yours?

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