Default value for capture in regular expression

redhotpenguin has asked for the wisdom of the Perl Monks concerning the following question:

Esteemed Monks,

While my regex tuits have been getting better, I'm faced with a challenge which is throwing me for a loop. I'm trying to extract values from a string (maybe a regex isn't the right way to do this).

The data I'm trying to parse:

my $string = <<STRING;
A Title
What:  A description of what it is
Date added:  January 16th, 2006 ( an optional source )
Data:  some data

Another Title
What:  Another description
Date added:  April 20th, 2005
Data:  some other data
STRING
[download]

I can parse most of this, but what I need to do is assign a default value for ( an optional source ) if the optional source is missing (the default value would be ''. Here's what I have so far:

my @fizzbin = ( $string =~ m{
    (\w+)\n                     # Grab the title
    What:\s+([^\n]+)\n          # Grab the what
    Date\sadded:\s+([^\(^\n]+)  # Date added
    (\([^\n]+\))\n              # Optional source
                                # I want to use a default
                                # value if nothing is           
                                # captured
    Data:\s+([^\n]+)\n          # Grab the data
}xmgs);
[download]

This regex works fine if I don't try to grab the optional source, it returns an array which is not ideal but provides the data I'm looking for ( $title1, $what1, $date1, $data1, $title2, $what2...) I don't know how to assign a default value to the source capture - any advice on that appreciated, including alternate methods of parsing here.

Thanks in advance.

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Re: Default value for capture in regular expression by ikegami (Patriarch) on Jan 16, 2006 at 20:55 UTC
Consider `'ab' =~ /a(z?)b/ # Returns ''. 'ab' =~ /a(z)?b/ # Returns undef.` [download] So `($[^\n]+$)` is optional? Then let's add a `?` as follows: `my @fizzbin = ( $string =~ m{ (\w+)\n # Grab the title What:\s+([^\n]+)\n # Grab the what Date\sadded:\s+([^$^\n]+) # Date added (\([^\n]+$)?\n # Optional source Data:\s+([^\n]+)\n # Grab the data }xmgs);` [download] After looking at my first snippet, we determine that the source will be `undef` if the source is omitted. Let's rearrange your code to use named variables like you wanted, and let's integrate the default. We get the following: `while ( my ($title, $what, $date, $source, $data) = $string =~ m{ (\w+)\n # Grab the title What:\s+([^\n]+)\n # Grab the what Date\sadded:\s+([^$^\n]+) # Date added (\([^\n]+$)?\n # Optional source Data:\s+([^\n]+)\n # Grab the data }xmgs ) { $source = '' if not defined $source; # Use default ''. ... }` [download] By the way, the `m` modifier is useless, since you don't use `^` or `$`; the `s` modifier is also useless, since you don't use `.`; and `Date\sadded` could be replaced with `Date[ ]added` to improve readability.	[reply] [d/l] [select]
Re: Default value for capture in regular expression by ww (Archbishop) on Jan 16, 2006 at 20:47 UTC
You might capture the optional source together with the date in a first pass, and then, inside an compound existence test, s/// such source data as does exist to $data1 while using the else clause to insert "No source indicated" or whatever as the new value for data1. Inelegant, and maybe not efficient, but sometimes 2 regexen are better than one.	[reply]
Re: Default value for capture in regular expression by bmcnett (Novice) on Jan 17, 2006 at 04:19 UTC
In cases like these I forego regexps for some kind of data language, such as Perl itself or JSON. I'd set up my defaults as a hash, then read a JSON object from a string (or a file, or wherever), then do a "hash slice" to override the defaults. `my %stuff = ( Title => 'some default', What => 'some default', 'Date added' => 'some default', Data => 'some default' ); use JSON; my $newstuff = jsonToObj( <<STRING ); { "Title": "Gone With The Wind", "Date added": "January 16th, 2006 ( an optional source )" } STRING @stuff{keys %$newstuff} = values %$newstuff;` [download]	[reply] [d/l]
Re: Default value for capture in regular expression by blazar (Canon) on Jan 17, 2006 at 08:45 UTC
I wouldn't do that with a (single) regex at all. There are tons of other ways: in particular in this case your data seems fairly regular. Thus one possibility (and just one amongst the may WTDI) may be: read in by paragraphs; split on `\n`; parse the single lines to get, say, `($data, $value)`; use a hash: `my %defult = (what => 'wtf?', date => 'no date', data => 'no data'); # ... unless (exists $default{$data}) { warn "unexpected field: $data'"; # ... } $value \|\|= $default{$data};` [download] Of course `//=` would be better suited for this, but since we currently don't have it yet unless we installed a patch, `\|\|=` should be enough. If it is not or in case of doubt, use your own defined test instead.	[reply] [d/l] [select]