Perl RegEx: Limiting the pattern to only the first occurrence of a character

Question

I am trying to extract the content of a date element from many ill-formed sgml documents. For instance, the document can contain a simple date element like

<DATE>4th July 1936</DATE>

or

<DATE blaAttrib="89787adjd98d9">4th July 1936</DATE>

but can also as hairy as:

<DATE blaAttrib="89787adjd98d9">4th July 1936
<EM>spanned across multiple lines and EM element inside DATE</EM></DATE>

The aim is to get the "4th July 1936". Since the files are not big, I chose to read the whole content into a variable and do the regex. The following is the snippet of my Perl code:

{
    local $/ = undef;
    open FILE, "$file" or die "Couldn't open file: $!";
    $fileContent = <FILE>;
    close FILE;

    if ( $fileContent =~ m/<DATE(.*)>(.*)<\/DATE>/)
    {
        # $2 should contain the "4th July 1936" but it did not.
    }
}

Unfortunately the regex does not work for the hairy example. This is because inside the <DATE> there is an <EM> element and it also spans multiple lines.

Can any kind soul give me some pointers, directions, or clues?

Thanks heaps!

Answer 1

Use an XML parser if you can.

But from your example, probably you could try

if ($fileContent =~ m/<DATE[^>]*>([^<]+)/) {
  # use $1 here
  # you may need to strip new lines
}

Answer 2

There is not any way to use regex over multiple lines, but you can use a little trick. If files aren't to big, as you have mentioned, you can first replace all '\n' characters with some value (NEW_LINE or something like that), or you can delete them and then use your pattern.

Answer 3

Use an HTML parser.

Use an HTML parser.

Please, use an HTML parser.

But for a regex, I'd try

<DATE(.*?)>(.*)<\/DATE>

which should be faster than KennyTM's alternative... By the way, why are you capturing that second group?

Answer 4

If the date format is fixed, you might want to use something like this:

m/<DATE(.*)>([0-9]+(st|nd|rd|th)\s(January|February|March|April|May|June|July|August|September|October|November|December)\s[0-9]+)(.*)<\/DATE>/

Answer 5

instead of matching .*, you should match "everything that is not an anchor"

ie :

 if($string =~ /^<DATE[^>]*>([^<]+)</){

there, $1 is your date

Answer 6

You should use non greedy matching and the modifier s to make . match newline

my @l = (
'<DATE>4th July 1936</DATE>',
'<DATE blaAttrib="89787adjd98d9">4th July 1936</DATE>',
'<DATE blaAttrib="89787adjd98d9">4th July 1936
<EM>spanned across multiple lines and EM element inside DATE</EM></DATE>'
);

foreach(@l) {
  /^<DATE.*?>(.*?)</s && print $1;
}

output:

4th July 1936
4th July 1936
4th July 1936

Answer 7

Even your "hairy" example can be reduced to a similar type. If you are always going to have 1) the actual date on the same line as the start tag--and 2) that's all you want--it doesn't matter where the end tag is.

$fileContent =~ m/<DATE([^>]*)>\s*(\d+\p{Alpha}+\s+\p{Alpha}+\s+\d{4})/

is always going to work. (If you're not going to find '>' in the tag, then it's a good idea to not cause so much backtracking after .* eats up your entire line, causes the expression to fail and then has to give back and check, give back and check, ...)