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| For 2023 (and 2024 ...) - we are now fully retired from IT training. We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk. Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active! I am also active in many other area and still look after a lot of web sites - you can find an index ((here)) |
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Using LWP to write Web Clients
As well as running server side scripts (through CGI, modPerl or ASP) and many uses independent of networks, Perl can be used to write server processes, and client processes too. "Why do I want to write my own browser?" you will ask. The answer, of course, is that you don't ... but you might well want to write an application to mimic a browser as it collects information from a server using HTTP. Let's say, for example, that I wish to write a Perl program to convert a price in one currency into another currency, using the current "spot" exchange rates. There's a suitable table available at: http://www.ecb.int/home/eurofxref.htm and it's updated daily. [Like all websites, ecb has just changed ;-( ... they now provide an XML file that's specifically intended for applications like this one :-) - it's at http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml] Although we could write a program using our own connections and sockets, it's much easier to use Perl's LWP module to do so - here are the sort of results we might get: $ ecbgrab AUD 1.693 Australian dollar BGN 1.951 Bulgarian lev CAD 1.371 Canadian dollar CHF 1.478 Swiss franc CYP 0.576 Cyprus pound CZK 31.910 Czech koruna DKK 7.428 Danish krone EEK 15.647 Estonian kroon EUR 1.000 European euros GBP 0.610 Pound sterling HKD 6.732 Hong Kong dollar HUF 243.000 Hungarian forint ISK 88.650 Icelandic krona JPY 115.660 Japanese yen KRW 1133.040 South Korean won LTL 3.453 Lithuanian litas LVL 0.555 Latvian lat MTL 0.398 Maltese lira NOK 7.840 Norwegian krone NZD 2.068 New Zealand dollar PLN 3.585 Polish zloty ROL 27700.000 Romanian leu SEK 9.188 Swedish krona SGD 1.585 Singaporean dollar SIT 222.598 Slovenian tolar SKK 42.470 Slovakian koruna TRL 1133000.000 Turkish lira USD 0.863 US dollar ZAR 9.931 South African rand Please enter an amount to convert (e.g. 290.00 GBP) ... 19.99 GBP into what currency ... NOK 19.99 Pound sterling converts to 256.94 Norwegian krone (GBP to NOK at 12.8533) $ Here's the program: #!/usr/bin/perl use LWP::UserAgent; $agent = LWP::UserAgent->new; $agent->agent("Well House Consultants/$0 "); $req = HTTP::Request->new(GET => "http://www.ecb.int/home/eurofxref.htm"); $res = $agent->request($req); $page = $res->content; %currencies = ("EUR","European euros",($page =~ />([A-Z]{3})<.*?>\s*<.*?>(.*?)</gs)); %rates = ("EUR",1,($page =~ />([A-Z]{3})<.*?>\s*<.*?>.*?<.*?>\s*<.*?>(.*?)</gs)); foreach $c (sort keys %currencies) { printf ("%4s %12.3f %s\n",$c,$rates{$c},$currencies{$c}); } print "Please enter an amount to convert (e.g. 290.00 GBP) ... "; chop ($yousaid = <STDIN>); if ($yousaid) { die ("invalid entry\n") unless(($amount,$incurr) = ($yousaid =~ /(.*?)\s*([A-Z]{3})$/)) ; die ("not a know currency\n") unless ($rates{$incurr}); print "into what currency ... "; chop ($yousaid = <STDIN>); die ("invalid entry\n") unless(($outcurr) = ($yousaid =~ /([A-Z]{3})$/)) ; die ("not a know currency\n") unless ($rates{$outcurr}); $becomes = $amount / $rates{$incurr} * $rates{$outcurr}; $erate = 1.0 / $rates{$incurr} * $rates{$outcurr}; printf ("%.2f %s converts to %.2f %s (%s to %s at %.4f)\n", $amount, $currencies{$incurr}, $becomes, $currencies{$outcurr}, $incurr, $outcurr, $erate); } We've used the LWP::UserAgent module from the CPAN - Browsers are know as "User Agents" in case you were wondering. We've given the User Agent a program name so that the server knows what type of agent (i.e. what model or browser) we are, and we've then made up a GET request and submitted it. There are other modules such as HTML-Parser available to help you parse the response, although in this particular case the response is in a straightforward enough format, so we've just used regular expressions for the job. A NOTE OF CAUTION Web servers were designed to supply information to browsers at the request of human users. Such accesses are relatively sporadic in computing terms, even for an enthusiastic user, so that lots of users can all be accessing the same web site in the same period of time and the web server can cope. If you write a client program that reaps a very large number of pages as fast as it can, it's quite possible that you'll overload the server or the internet connection to it, and your effort may be seen as unwelcome - it may even be classified as a "denial of service" attack. There are two rules to note if you are going to be looking for many pages, or if you are going to be looking regularly. Firstly, you should look at a file called robots.txt which should be in the home directory on the web server; this file contains information placed on the web server by the web site administrator, and tells robots where they are not welcome. Second, if you require more than one page from a server, you should pause between each page that you grab to give other users a chance of a look in. Chances are that if you have a major robotic program, you'll be looking at pages on many different sites so you don't actually have to slow your program down - just grab pages from each site in turn See also Perl on the Web course Please note that articles in this section of our
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Related Material
Web Application Deployment - XML, DTD, XSLT, XHTML and More [653] - () [1050] - () [1901] - () [2246] - () [2378] - () [2554] - () Handling XML in Perl [2378] - () [2555] - () [3874] - () Perl - Standard Web Modules [975] - () [2229] - () [2402] - () [2416] - () [3485] - () [4099] - () [4100] - () resource index - Perl Solutions centre home page You'll find shorter technical items at The Horse's Mouth and delegate's questions answered at the Opentalk forum. At Well House Consultants, we provide training courses on subjects such as Ruby, Lua, Perl, Python, Linux, C, C++, Tcl/Tk, Tomcat, PHP and MySQL. We're asked (and answer) many questions, and answers to those which are of general interest are published in this area of our site. |
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