Re: Regular expressions???

[Ray Blaak <blaak@infomatch.com>]

Michael Andersson <michael@ida.his.se> writes:


> I'm trying to write a simple XML-parser and I wonder how regular
> expressions in Ada works. Can I use variables as in Pearl so that these
> are assigned a value according to the values found in the string. Say
> for example that I have a file looking like this:
> <Name="Michael" Phone="7980438" Age="22"/>
> I want to extract Michael, 7989438 and 22 from the string above and
> assign these to some variables. Is it possible to use regular
> expressions in Ada to do this or do I have to use procedure/functions
> from the Ada.Strings package?

For this problem, it is in fact easier and faster to parse the XML properly to
extract the relevant bits. That is, make a pseudo-state machine that looks for
<, >, =, ", />, etc, performing callbacks or inserting into an XML tree
structure when the relevant bits are recognized.

If you are going to hunt around for a RE package in Ada, you can also hunt
around for an XML package. 

Otherwise, writing an XML parser is a lot easier than writing a RE processor.

Regular expressions are often mismused where a real parser would be more
appropriate. Consider an RE that can extract bits from this line:

  <Person Name="Michael" Phone="7980438" Age="22"/>

Fine. Now, would that RE work for this:

  <Person
     Name="Michael" 
     Phone="7980438"
     Age="22"/>

or this:

  <Person Age="22" Phone="7980438" Name="Michael" />

which are both equivalent semantically?

REs work best with line oriented fix-formatted data. Free-form data tends to
break REs.

-- 
Cheers,                                        The Rhythm is around me,
                                               The Rhythm has control.
Ray Blaak                                      The Rhythm is inside me,
blaak@infomatch.com                            The Rhythm has my soul.