mingw/html/pod/perlintro.html

   1 <?xml version="1.0" ?>
   2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   3 <html xmlns="http://www.w3.org/1999/xhtml">
   4 <head>
   5 <title>perlintro -- a brief introduction and overview of Perl</title>
   6 <meta http-equiv="content-type" content="text/html; charset=utf-8" />
   7 <link rev="made" href="mailto:" />
   8 </head>
   9
  10 <body style="background-color: white">
  11 <table border="0" width="100%" cellspacing="0" cellpadding="3">
  12 <tr><td class="block" style="background-color: #cccccc" valign="middle">
  13 <big><strong><span class="block">&nbsp;perlintro -- a brief introduction and overview of Perl</span></strong></big>
  14 </td></tr>
  15 </table>
  16
  17 <p><a name="__index__"></a></p>
  18 <!-- INDEX BEGIN -->
  19
  20 <ul>
  21
  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#description">DESCRIPTION</a></li>
  24         <ul>
  25
  26                 <li><a href="#what_is_perl">What is Perl?</a></li>
  27                 <li><a href="#running_perl_programs">Running Perl programs</a></li>
  28                 <li><a href="#basic_syntax_overview">Basic syntax overview</a></li>
  29                 <li><a href="#perl_variable_types">Perl variable types</a></li>
  30                 <li><a href="#variable_scoping">Variable scoping</a></li>
  31                 <li><a href="#conditional_and_looping_constructs">Conditional and looping constructs</a></li>
  32                 <li><a href="#builtin_operators_and_functions">Builtin operators and functions</a></li>
  33                 <li><a href="#files_and_i_o">Files and I/O</a></li>
  34                 <li><a href="#regular_expressions">Regular expressions</a></li>
  35                 <li><a href="#writing_subroutines">Writing subroutines</a></li>
  36                 <li><a href="#oo_perl">OO Perl</a></li>
  37                 <li><a href="#using_perl_modules">Using Perl modules</a></li>
  38         </ul>
  39
  40         <li><a href="#author">AUTHOR</a></li>
  41 </ul>
  42 <!-- INDEX END -->
  43
  44 <hr />
  45 <p>
  46 </p>
  47 <h1><a name="name">NAME</a></h1>
  48 <p>perlintro -- a brief introduction and overview of Perl</p>
  49 <p>
  50 </p>
  51 <hr />
  52 <h1><a name="description">DESCRIPTION</a></h1>
  53 <p>This document is intended to give you a quick overview of the Perl
  54 programming language, along with pointers to further documentation.  It
  55 is intended as a ``bootstrap'' guide for those who are new to the
  56 language, and provides just enough information for you to be able to
  57 read other peoples' Perl and understand roughly what it's doing, or
  58 write your own simple scripts.</p>
  59 <p>This introductory document does not aim to be complete.  It does not
  60 even aim to be entirely accurate.  In some cases perfection has been
  61 sacrificed in the goal of getting the general idea across.  You are
  62 <em>strongly</em> advised to follow this introduction with more information
  63 from the full Perl manual, the table of contents to which can be found
  64 in <a href="file://C|\msysgit\mingw\html/pod/perltoc.html">the perltoc manpage</a>.</p>
  65 <p>Throughout this document you'll see references to other parts of the
  66 Perl documentation.  You can read that documentation using the <code>perldoc</code>
  67 command or whatever method you're using to read this document.</p>
  68 <p>
  69 </p>
  70 <h2><a name="what_is_perl">What is Perl?</a></h2>
  71 <p>Perl is a general-purpose programming language originally developed for
  72 text manipulation and now used for a wide range of tasks including
  73 system administration, web development, network programming, GUI
  74 development, and more.</p>
  75 <p>The language is intended to be practical (easy to use, efficient,
  76 complete) rather than beautiful (tiny, elegant, minimal).  Its major
  77 features are that it's easy to use, supports both procedural and
  78 object-oriented (OO) programming, has powerful built-in support for text
  79 processing, and has one of the world's most impressive collections of
  80 third-party modules.</p>
  81 <p>Different definitions of Perl are given in <a href="file://C|\msysgit\mingw\html/pod/perl.html">the perl manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perlfaq1.html">the perlfaq1 manpage</a> and
  82 no doubt other places.  From this we can determine that Perl is different
  83 things to different people, but that lots of people think it's at least
  84 worth writing about.</p>
  85 <p>
  86 </p>
  87 <h2><a name="running_perl_programs">Running Perl programs</a></h2>
  88 <p>To run a Perl program from the Unix command line:</p>
  89 <pre>
  90     perl progname.pl</pre>
  91 <p>Alternatively, put this as the first line of your script:</p>
  92 <pre>
  93     #!/usr/bin/env perl</pre>
  94 <p>... and run the script as <code>/path/to/script.pl</code>.  Of course, it'll need
  95 to be executable first, so <code>chmod 755 script.pl</code> (under Unix).</p>
  96 <p>For more information, including instructions for other platforms such as
  97 Windows and Mac OS, read <a href="file://C|\msysgit\mingw\html/pod/perlrun.html">the perlrun manpage</a>.</p>
  98 <p>
  99 </p>
 100 <h2><a name="basic_syntax_overview">Basic syntax overview</a></h2>
 101 <p>A Perl script or program consists of one or more statements.  These
 102 statements are simply written in the script in a straightforward
 103 fashion.  There is no need to have a <code>main()</code> function or anything of
 104 that kind.</p>
 105 <p>Perl statements end in a semi-colon:</p>
 106 <pre>
 107     print &quot;Hello, world&quot;;</pre>
 108 <p>Comments start with a hash symbol and run to the end of the line</p>
 109 <pre>
 110     # This is a comment</pre>
 111 <p>Whitespace is irrelevant:</p>
 112 <pre>
 113     print
 114         &quot;Hello, world&quot;
 115         ;</pre>
 116 <p>... except inside quoted strings:</p>
 117 <pre>
 118     # this would print with a linebreak in the middle
 119     print &quot;Hello
 120     world&quot;;</pre>
 121 <p>Double quotes or single quotes may be used around literal strings:</p>
 122 <pre>
 123     print &quot;Hello, world&quot;;
 124     print 'Hello, world';</pre>
 125 <p>However, only double quotes ``interpolate'' variables and special
 126 characters such as newlines (<code>\n</code>):</p>
 127 <pre>
 128     print &quot;Hello, $name\n&quot;;     # works fine
 129     print 'Hello, $name\n';     # prints $name\n literally</pre>
 130 <p>Numbers don't need quotes around them:</p>
 131 <pre>
 132     print 42;</pre>
 133 <p>You can use parentheses for functions' arguments or omit them
 134 according to your personal taste.  They are only required
 135 occasionally to clarify issues of precedence.</p>
 136 <pre>
 137     print(&quot;Hello, world\n&quot;);
 138     print &quot;Hello, world\n&quot;;</pre>
 139 <p>More detailed information about Perl syntax can be found in <a href="file://C|\msysgit\mingw\html/pod/perlsyn.html">the perlsyn manpage</a>.</p>
 140 <p>
 141 </p>
 142 <h2><a name="perl_variable_types">Perl variable types</a></h2>
 143 <p>Perl has three main variable types: scalars, arrays, and hashes.</p>
 144 <dl>
 145 <dt><strong><a name="item_scalars">Scalars</a></strong>
 146
 147 <dd>
 148 <p>A scalar represents a single value:</p>
 149 </dd>
 150 <dd>
 151 <pre>
 152     my $animal = &quot;camel&quot;;
 153     my $answer = 42;</pre>
 154 </dd>
 155 <dd>
 156 <p>Scalar values can be strings, integers or floating point numbers, and Perl
 157 will automatically convert between them as required.  There is no need
 158 to pre-declare your variable types.</p>
 159 </dd>
 160 <dd>
 161 <p>Scalar values can be used in various ways:</p>
 162 </dd>
 163 <dd>
 164 <pre>
 165     print $animal;
 166     print &quot;The animal is $animal\n&quot;;
 167     print &quot;The square of $answer is &quot;, $answer * $answer, &quot;\n&quot;;</pre>
 168 </dd>
 169 <dd>
 170 <p>There are a number of ``magic'' scalars with names that look like
 171 punctuation or line noise.  These special variables are used for all
 172 kinds of purposes, and are documented in <a href="file://C|\msysgit\mingw\html/pod/perlvar.html">the perlvar manpage</a>.  The only one you
 173 need to know about for now is <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$_</code></a> which is the ``default variable''.
 174 It's used as the default argument to a number of functions in Perl, and
 175 it's set implicitly by certain looping constructs.</p>
 176 </dd>
 177 <dd>
 178 <pre>
 179     print;          # prints contents of $_ by default</pre>
 180 </dd>
 181 </li>
 182 <dt><strong><a name="item_arrays">Arrays</a></strong>
 183
 184 <dd>
 185 <p>An array represents a list of values:</p>
 186 </dd>
 187 <dd>
 188 <pre>
 189     my @animals = (&quot;camel&quot;, &quot;llama&quot;, &quot;owl&quot;);
 190     my @numbers = (23, 42, 69);
 191     my @mixed   = (&quot;camel&quot;, 42, 1.23);</pre>
 192 </dd>
 193 <dd>
 194 <p>Arrays are zero-indexed.  Here's how you get at elements in an array:</p>
 195 </dd>
 196 <dd>
 197 <pre>
 198     print $animals[0];              # prints &quot;camel&quot;
 199     print $animals[1];              # prints &quot;llama&quot;</pre>
 200 </dd>
 201 <dd>
 202 <p>The special variable <code>$#array</code> tells you the index of the last element
 203 of an array:</p>
 204 </dd>
 205 <dd>
 206 <pre>
 207     print $mixed[$#mixed];       # last element, prints 1.23</pre>
 208 </dd>
 209 <dd>
 210 <p>You might be tempted to use <code>$#array + 1</code> to tell you how many items there
 211 are in an array.  Don't bother.  As it happens, using <code>@array</code> where Perl
 212 expects to find a scalar value (``in scalar context'') will give you the number
 213 of elements in the array:</p>
 214 </dd>
 215 <dd>
 216 <pre>
 217     if (@animals &lt; 5) { ... }</pre>
 218 </dd>
 219 <dd>
 220 <p>The elements we're getting from the array start with a <code>$</code> because
 221 we're getting just a single value out of the array -- you ask for a scalar,
 222 you get a scalar.</p>
 223 </dd>
 224 <dd>
 225 <p>To get multiple values from an array:</p>
 226 </dd>
 227 <dd>
 228 <pre>
 229     @animals[0,1];                  # gives (&quot;camel&quot;, &quot;llama&quot;);
 230     @animals[0..2];                 # gives (&quot;camel&quot;, &quot;llama&quot;, &quot;owl&quot;);
 231     @animals[1..$#animals];         # gives all except the first element</pre>
 232 </dd>
 233 <dd>
 234 <p>This is called an ``array slice''.</p>
 235 </dd>
 236 <dd>
 237 <p>You can do various useful things to lists:</p>
 238 </dd>
 239 <dd>
 240 <pre>
 241     my @sorted    = sort @animals;
 242     my @backwards = reverse @numbers;</pre>
 243 </dd>
 244 <dd>
 245 <p>There are a couple of special arrays too, such as <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__argv"><code>@ARGV</code></a> (the command
 246 line arguments to your script) and <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>@_</code></a> (the arguments passed to a
 247 subroutine).  These are documented in <a href="file://C|\msysgit\mingw\html/pod/perlvar.html">the perlvar manpage</a>.</p>
 248 </dd>
 249 </li>
 250 <dt><strong><a name="item_hashes">Hashes</a></strong>
 251
 252 <dd>
 253 <p>A hash represents a set of key/value pairs:</p>
 254 </dd>
 255 <dd>
 256 <pre>
 257     my %fruit_color = (&quot;apple&quot;, &quot;red&quot;, &quot;banana&quot;, &quot;yellow&quot;);</pre>
 258 </dd>
 259 <dd>
 260 <p>You can use whitespace and the <code>=&gt;</code> operator to lay them out more
 261 nicely:</p>
 262 </dd>
 263 <dd>
 264 <pre>
 265     my %fruit_color = (
 266         apple  =&gt; &quot;red&quot;,
 267         banana =&gt; &quot;yellow&quot;,
 268     );</pre>
 269 </dd>
 270 <dd>
 271 <p>To get at hash elements:</p>
 272 </dd>
 273 <dd>
 274 <pre>
 275     $fruit_color{&quot;apple&quot;};           # gives &quot;red&quot;</pre>
 276 </dd>
 277 <dd>
 278 <p>You can get at lists of keys and values with <code>keys()</code> and
 279 <code>values()</code>.</p>
 280 </dd>
 281 <dd>
 282 <pre>
 283     my @fruits = keys %fruit_colors;
 284     my @colors = values %fruit_colors;</pre>
 285 </dd>
 286 <dd>
 287 <p>Hashes have no particular internal order, though you can sort the keys
 288 and loop through them.</p>
 289 </dd>
 290 <dd>
 291 <p>Just like special scalars and arrays, there are also special hashes.
 292 The most well known of these is <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__env"><code>%ENV</code></a> which contains environment
 293 variables.  Read all about it (and other special variables) in
 294 <a href="file://C|\msysgit\mingw\html/pod/perlvar.html">the perlvar manpage</a>.</p>
 295 </dd>
 296 </li>
 297 </dl>
 298 <p>Scalars, arrays and hashes are documented more fully in <a href="file://C|\msysgit\mingw\html/pod/perldata.html">the perldata manpage</a>.</p>
 299 <p>More complex data types can be constructed using references, which allow
 300 you to build lists and hashes within lists and hashes.</p>
 301 <p>A reference is a scalar value and can refer to any other Perl data
 302 type. So by storing a reference as the value of an array or hash
 303 element, you can easily create lists and hashes within lists and
 304 hashes. The following example shows a 2 level hash of hash
 305 structure using anonymous hash references.</p>
 306 <pre>
 307     my $variables = {
 308         scalar  =&gt;  {
 309                      description =&gt; &quot;single item&quot;,
 310                      sigil =&gt; '$',
 311                     },
 312         array   =&gt;  {
 313                      description =&gt; &quot;ordered list of items&quot;,
 314                      sigil =&gt; '@',
 315                     },
 316         hash    =&gt;  {
 317                      description =&gt; &quot;key/value pairs&quot;,
 318                      sigil =&gt; '%',
 319                     },
 320     };</pre>
 321 <pre>
 322     print &quot;Scalars begin with a $variables-&gt;{'scalar'}-&gt;{'sigil'}\n&quot;;</pre>
 323 <p>Exhaustive information on the topic of references can be found in
 324 <a href="file://C|\msysgit\mingw\html/pod/perlreftut.html">the perlreftut manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perllol.html">the perllol manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a> and <a href="file://C|\msysgit\mingw\html/pod/perldsc.html">the perldsc manpage</a>.</p>
 325 <p>
 326 </p>
 327 <h2><a name="variable_scoping">Variable scoping</a></h2>
 328 <p>Throughout the previous section all the examples have used the syntax:</p>
 329 <pre>
 330     my $var = &quot;value&quot;;</pre>
 331 <p>The <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my"><code>my</code></a> is actually not required; you could just use:</p>
 332 <pre>
 333     $var = &quot;value&quot;;</pre>
 334 <p>However, the above usage will create global variables throughout your
 335 program, which is bad programming practice.  <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my"><code>my</code></a> creates lexically
 336 scoped variables instead.  The variables are scoped to the block
 337 (i.e. a bunch of statements surrounded by curly-braces) in which they
 338 are defined.</p>
 339 <pre>
 340     my $a = &quot;foo&quot;;
 341     if ($some_condition) {
 342         my $b = &quot;bar&quot;;
 343         print $a;           # prints &quot;foo&quot;
 344         print $b;           # prints &quot;bar&quot;
 345     }
 346     print $a;               # prints &quot;foo&quot;
 347     print $b;               # prints nothing; $b has fallen out of scope</pre>
 348 <p>Using <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my"><code>my</code></a> in combination with a <code>use strict;</code> at the top of
 349 your Perl scripts means that the interpreter will pick up certain common
 350 programming errors.  For instance, in the example above, the final
 351 <code>print $b</code> would cause a compile-time error and prevent you from
 352 running the program.  Using <code>strict</code> is highly recommended.</p>
 353 <p>
 354 </p>
 355 <h2><a name="conditional_and_looping_constructs">Conditional and looping constructs</a></h2>
 356 <p>Perl has most of the usual conditional and looping constructs except for
 357 case/switch (but if you really want it, there is a Switch module in Perl
 358 5.8 and newer, and on CPAN. See the section on modules, below, for more
 359 information about modules and CPAN).</p>
 360 <p>The conditions can be any Perl expression.  See the list of operators in
 361 the next section for information on comparison and boolean logic operators,
 362 which are commonly used in conditional statements.</p>
 363 <dl>
 364 <dt><strong><a name="item_if">if</a></strong>
 365
 366 <dd>
 367 <pre>
 368     if ( condition ) {
 369         ...
 370     } elsif ( other condition ) {
 371         ...
 372     } else {
 373         ...
 374     }</pre>
 375 </dd>
 376 <dd>
 377 <p>There's also a negated version of it:</p>
 378 </dd>
 379 <dd>
 380 <pre>
 381     unless ( condition ) {
 382         ...
 383     }</pre>
 384 </dd>
 385 <dd>
 386 <p>This is provided as a more readable version of <a href="#item_if"><code>if (!condition)</code></a>.</p>
 387 </dd>
 388 <dd>
 389 <p>Note that the braces are required in Perl, even if you've only got one
 390 line in the block.  However, there is a clever way of making your one-line
 391 conditional blocks more English like:</p>
 392 </dd>
 393 <dd>
 394 <pre>
 395     # the traditional way
 396     if ($zippy) {
 397         print &quot;Yow!&quot;;
 398     }</pre>
 399 </dd>
 400 <dd>
 401 <pre>
 402     # the Perlish post-condition way
 403     print &quot;Yow!&quot; if $zippy;
 404     print &quot;We have no bananas&quot; unless $bananas;</pre>
 405 </dd>
 406 <dt><strong><a name="item_while">while</a></strong>
 407
 408 <dd>
 409 <pre>
 410     while ( condition ) {
 411         ...
 412     }</pre>
 413 </dd>
 414 <dd>
 415 <p>There's also a negated version, for the same reason we have <code>unless</code>:</p>
 416 </dd>
 417 <dd>
 418 <pre>
 419     until ( condition ) {
 420         ...
 421     }</pre>
 422 </dd>
 423 <dd>
 424 <p>You can also use <a href="#item_while"><code>while</code></a> in a post-condition:</p>
 425 </dd>
 426 <dd>
 427 <pre>
 428     print &quot;LA LA LA\n&quot; while 1;          # loops forever</pre>
 429 </dd>
 430 <dt><strong><a name="item_for">for</a></strong>
 431
 432 <dd>
 433 <p>Exactly like C:</p>
 434 </dd>
 435 <dd>
 436 <pre>
 437     for ($i=0; $i &lt;= $max; $i++) {
 438         ...
 439     }</pre>
 440 </dd>
 441 <dd>
 442 <p>The C style for loop is rarely needed in Perl since Perl provides
 443 the more friendly list scanning <a href="#item_foreach"><code>foreach</code></a> loop.</p>
 444 </dd>
 445 </li>
 446 <dt><strong><a name="item_foreach">foreach</a></strong>
 447
 448 <dd>
 449 <pre>
 450     foreach (@array) {
 451         print &quot;This element is $_\n&quot;;
 452     }</pre>
 453 </dd>
 454 <dd>
 455 <pre>
 456     # you don't have to use the default $_ either...
 457     foreach my $key (keys %hash) {
 458         print &quot;The value of $key is $hash{$key}\n&quot;;
 459     }</pre>
 460 </dd>
 461 </dl>
 462 <p>For more detail on looping constructs (and some that weren't mentioned in
 463 this overview) see <a href="file://C|\msysgit\mingw\html/pod/perlsyn.html">the perlsyn manpage</a>.</p>
 464 <p>
 465 </p>
 466 <h2><a name="builtin_operators_and_functions">Builtin operators and functions</a></h2>
 467 <p>Perl comes with a wide selection of builtin functions.  Some of the ones
 468 we've already seen include <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_print"><code>print</code></a>, <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_sort"><code>sort</code></a> and <code>reverse</code>.  A list of
 469 them is given at the start of <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html">the perlfunc manpage</a> and you can easily read
 470 about any given function by using <code>perldoc -f functionname</code>.</p>
 471 <p>Perl operators are documented in full in <a href="file://C|\msysgit\mingw\html/pod/perlop.html">the perlop manpage</a>, but here are a few
 472 of the most common ones:</p>
 473 <dl>
 474 <dt><strong><a name="item_arithmetic">Arithmetic</a></strong>
 475
 476 <dd>
 477 <pre>
 478     +   addition
 479     -   subtraction
 480     *   multiplication
 481     /   division</pre>
 482 </dd>
 483 <dt><strong><a name="item_numeric_comparison">Numeric comparison</a></strong>
 484
 485 <dd>
 486 <pre>
 487     ==  equality
 488     !=  inequality
 489     &lt;   less than
 490     &gt;   greater than
 491     &lt;=  less than or equal
 492     &gt;=  greater than or equal</pre>
 493 </dd>
 494 <dt><strong><a name="item_string_comparison">String comparison</a></strong>
 495
 496 <dd>
 497 <pre>
 498     eq  equality
 499     ne  inequality
 500     lt  less than
 501     gt  greater than
 502     le  less than or equal
 503     ge  greater than or equal</pre>
 504 </dd>
 505 <dd>
 506 <p>(Why do we have separate numeric and string comparisons?  Because we don't
 507 have special variable types, and Perl needs to know whether to sort
 508 numerically (where 99 is less than 100) or alphabetically (where 100 comes
 509 before 99).</p>
 510 </dd>
 511 <dt><strong><a name="item_boolean_logic">Boolean logic</a></strong>
 512
 513 <dd>
 514 <pre>
 515     &amp;&amp;  and
 516     ||  or
 517     !   not</pre>
 518 </dd>
 519 <dd>
 520 <p>(<code>and</code>, <code>or</code> and <code>not</code> aren't just in the above table as descriptions
 521 of the operators -- they're also supported as operators in their own
 522 right.  They're more readable than the C-style operators, but have
 523 different precedence to <code>&amp;&amp;</code> and friends.  Check <a href="file://C|\msysgit\mingw\html/pod/perlop.html">the perlop manpage</a> for more
 524 detail.)</p>
 525 </dd>
 526 <dt><strong><a name="item_miscellaneous">Miscellaneous</a></strong>
 527
 528 <dd>
 529 <pre>
 530     =   assignment
 531     .   string concatenation
 532     x   string multiplication
 533     ..  range operator (creates a list of numbers)</pre>
 534 </dd>
 535 </dl>
 536 <p>Many operators can be combined with a <code>=</code> as follows:</p>
 537 <pre>
 538     $a += 1;        # same as $a = $a + 1
 539     $a -= 1;        # same as $a = $a - 1
 540     $a .= &quot;\n&quot;;     # same as $a = $a . &quot;\n&quot;;</pre>
 541 <p>
 542 </p>
 543 <h2><a name="files_and_i_o">Files and I/O</a></h2>
 544 <p>You can open a file for input or output using the <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open()</code></a> function.
 545 It's documented in extravagant detail in <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html">the perlfunc manpage</a> and <a href="file://C|\msysgit\mingw\html/pod/perlopentut.html">the perlopentut manpage</a>,
 546 but in short:</p>
 547 <pre>
 548     open(INFILE,  &quot;input.txt&quot;)   or die &quot;Can't open input.txt: $!&quot;;
 549     open(OUTFILE, &quot;&gt;output.txt&quot;) or die &quot;Can't open output.txt: $!&quot;;
 550     open(LOGFILE, &quot;&gt;&gt;my.log&quot;)    or die &quot;Can't open logfile: $!&quot;;</pre>
 551 <p>You can read from an open filehandle using the <code>&lt;&gt;</code> operator.  In
 552 scalar context it reads a single line from the filehandle, and in list
 553 context it reads the whole file in, assigning each line to an element of
 554 the list:</p>
 555 <pre>
 556     my $line  = &lt;INFILE&gt;;
 557     my @lines = &lt;INFILE&gt;;</pre>
 558 <p>Reading in the whole file at one time is called slurping. It can
 559 be useful but it may be a memory hog. Most text file processing
 560 can be done a line at a time with Perl's looping constructs.</p>
 561 <p>The <code>&lt;&gt;</code> operator is most often seen in a <a href="#item_while"><code>while</code></a> loop:</p>
 562 <pre>
 563     while (&lt;INFILE&gt;) {     # assigns each line in turn to $_
 564         print &quot;Just read in this line: $_&quot;;
 565     }</pre>
 566 <p>We've already seen how to print to standard output using <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_print"><code>print()</code></a>.
 567 However, <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_print"><code>print()</code></a> can also take an optional first argument specifying
 568 which filehandle to print to:</p>
 569 <pre>
 570     print STDERR &quot;This is your final warning.\n&quot;;
 571     print OUTFILE $record;
 572     print LOGFILE $logmessage;</pre>
 573 <p>When you're done with your filehandles, you should <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_close"><code>close()</code></a> them
 574 (though to be honest, Perl will clean up after you if you forget):</p>
 575 <pre>
 576     close INFILE;</pre>
 577 <p>
 578 </p>
 579 <h2><a name="regular_expressions">Regular expressions</a></h2>
 580 <p>Perl's regular expression support is both broad and deep, and is the
 581 subject of lengthy documentation in <a href="file://C|\msysgit\mingw\html/pod/perlrequick.html">the perlrequick manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perlretut.html">the perlretut manpage</a>, and
 582 elsewhere.  However, in short:</p>
 583 <dl>
 584 <dt><strong><a name="item_simple_matching">Simple matching</a></strong>
 585
 586 <dd>
 587 <pre>
 588     if (/foo/)       { ... }  # true if $_ contains &quot;foo&quot;
 589     if ($a =~ /foo/) { ... }  # true if $a contains &quot;foo&quot;</pre>
 590 </dd>
 591 <dd>
 592 <p>The <code>//</code> matching operator is documented in <a href="file://C|\msysgit\mingw\html/pod/perlop.html">the perlop manpage</a>.  It operates on
 593 <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$_</code></a> by default, or can be bound to another variable using the <code>=~</code>
 594 binding operator (also documented in <a href="file://C|\msysgit\mingw\html/pod/perlop.html">the perlop manpage</a>).</p>
 595 </dd>
 596 <dt><strong><a name="item_simple_substitution">Simple substitution</a></strong>
 597
 598 <dd>
 599 <pre>
 600     s/foo/bar/;               # replaces foo with bar in $_
 601     $a =~ s/foo/bar/;         # replaces foo with bar in $a
 602     $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $a</pre>
 603 </dd>
 604 <dd>
 605 <p>The <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_s_"><code>s///</code></a> substitution operator is documented in <a href="file://C|\msysgit\mingw\html/pod/perlop.html">the perlop manpage</a>.</p>
 606 </dd>
 607 <dt><strong><a name="item_more_complex_regular_expressions">More complex regular expressions</a></strong>
 608
 609 <dd>
 610 <p>You don't just have to match on fixed strings.  In fact, you can match
 611 on just about anything you could dream of by using more complex regular
 612 expressions.  These are documented at great length in <a href="file://C|\msysgit\mingw\html/pod/perlre.html">the perlre manpage</a>, but for
 613 the meantime, here's a quick cheat sheet:</p>
 614 </dd>
 615 <dd>
 616 <pre>
 617     .                   a single character
 618     \s                  a whitespace character (space, tab, newline)
 619     \S                  non-whitespace character
 620     \d                  a digit (0-9)
 621     \D                  a non-digit
 622     \w                  a word character (a-z, A-Z, 0-9, _)
 623     \W                  a non-word character
 624     [aeiou]             matches a single character in the given set
 625     [^aeiou]            matches a single character outside the given set
 626     (foo|bar|baz)       matches any of the alternatives specified</pre>
 627 </dd>
 628 <dd>
 629 <pre>
 630     ^                   start of string
 631     $                   end of string</pre>
 632 </dd>
 633 <dd>
 634 <p>Quantifiers can be used to specify how many of the previous thing you
 635 want to match on, where ``thing'' means either a literal character, one
 636 of the metacharacters listed above, or a group of characters or
 637 metacharacters in parentheses.</p>
 638 </dd>
 639 <dd>
 640 <pre>
 641     *                   zero or more of the previous thing
 642     +                   one or more of the previous thing
 643     ?                   zero or one of the previous thing
 644     {3}                 matches exactly 3 of the previous thing
 645     {3,6}               matches between 3 and 6 of the previous thing
 646     {3,}                matches 3 or more of the previous thing</pre>
 647 </dd>
 648 <dd>
 649 <p>Some brief examples:</p>
 650 </dd>
 651 <dd>
 652 <pre>
 653     /^\d+/              string starts with one or more digits
 654     /^$/                nothing in the string (start and end are adjacent)
 655     /(\d\s){3}/         a three digits, each followed by a whitespace
 656                         character (eg &quot;3 4 5 &quot;)
 657     /(a.)+/             matches a string in which every odd-numbered letter
 658                         is a (eg &quot;abacadaf&quot;)</pre>
 659 </dd>
 660 <dd>
 661 <pre>
 662     # This loop reads from STDIN, and prints non-blank lines:
 663     while (&lt;&gt;) {
 664         next if /^$/;
 665         print;
 666     }</pre>
 667 </dd>
 668 </li>
 669 <dt><strong><a name="item_parentheses_for_capturing">Parentheses for capturing</a></strong>
 670
 671 <dd>
 672 <p>As well as grouping, parentheses serve a second purpose.  They can be
 673 used to capture the results of parts of the regexp match for later use.
 674 The results end up in <code>$1</code>, <code>$2</code> and so on.</p>
 675 </dd>
 676 <dd>
 677 <pre>
 678     # a cheap and nasty way to break an email address up into parts</pre>
 679 </dd>
 680 <dd>
 681 <pre>
 682     if ($email =~ /([^@]+)@(.+)/) {
 683         print &quot;Username is $1\n&quot;;
 684         print &quot;Hostname is $2\n&quot;;
 685     }</pre>
 686 </dd>
 687 </li>
 688 <dt><strong><a name="item_other_regexp_features">Other regexp features</a></strong>
 689
 690 <dd>
 691 <p>Perl regexps also support backreferences, lookaheads, and all kinds of
 692 other complex details.  Read all about them in <a href="file://C|\msysgit\mingw\html/pod/perlrequick.html">the perlrequick manpage</a>,
 693 <a href="file://C|\msysgit\mingw\html/pod/perlretut.html">the perlretut manpage</a>, and <a href="file://C|\msysgit\mingw\html/pod/perlre.html">the perlre manpage</a>.</p>
 694 </dd>
 695 </li>
 696 </dl>
 697 <p>
 698 </p>
 699 <h2><a name="writing_subroutines">Writing subroutines</a></h2>
 700 <p>Writing subroutines is easy:</p>
 701 <pre>
 702     sub log {
 703         my $logmessage = shift;
 704         print LOGFILE $logmessage;
 705     }</pre>
 706 <p>What's that <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_shift"><code>shift</code></a>?  Well, the arguments to a subroutine are available
 707 to us as a special array called <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>@_</code></a> (see <a href="file://C|\msysgit\mingw\html/pod/perlvar.html">the perlvar manpage</a> for more on that).
 708 The default argument to the <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_shift"><code>shift</code></a> function just happens to be <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>@_</code></a>.
 709 So <code>my $logmessage = shift;</code> shifts the first item off the list of
 710 arguments and assigns it to <code>$logmessage</code>.</p>
 711 <p>We can manipulate <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>@_</code></a> in other ways too:</p>
 712 <pre>
 713     my ($logmessage, $priority) = @_;       # common
 714     my $logmessage = $_[0];                 # uncommon, and ugly</pre>
 715 <p>Subroutines can also return values:</p>
 716 <pre>
 717     sub square {
 718         my $num = shift;
 719         my $result = $num * $num;
 720         return $result;
 721     }</pre>
 722 <p>For more information on writing subroutines, see <a href="file://C|\msysgit\mingw\html/pod/perlsub.html">the perlsub manpage</a>.</p>
 723 <p>
 724 </p>
 725 <h2><a name="oo_perl">OO Perl</a></h2>
 726 <p>OO Perl is relatively simple and is implemented using references which
 727 know what sort of object they are based on Perl's concept of packages.
 728 However, OO Perl is largely beyond the scope of this document.
 729 Read <a href="file://C|\msysgit\mingw\html/pod/perlboot.html">the perlboot manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perltoot.html">the perltoot manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perltooc.html">the perltooc manpage</a> and <a href="file://C|\msysgit\mingw\html/pod/perlobj.html">the perlobj manpage</a>.</p>
 730 <p>As a beginning Perl programmer, your most common use of OO Perl will be
 731 in using third-party modules, which are documented below.</p>
 732 <p>
 733 </p>
 734 <h2><a name="using_perl_modules">Using Perl modules</a></h2>
 735 <p>Perl modules provide a range of features to help you avoid reinventing
 736 the wheel, and can be downloaded from CPAN ( <a href="http://www.cpan.org/">http://www.cpan.org/</a> ).  A
 737 number of popular modules are included with the Perl distribution
 738 itself.</p>
 739 <p>Categories of modules range from text manipulation to network protocols
 740 to database integration to graphics.  A categorized list of modules is
 741 also available from CPAN.</p>
 742 <p>To learn how to install modules you download from CPAN, read
 743 <a href="file://C|\msysgit\mingw\html/pod/perlmodinstall.html">the perlmodinstall manpage</a></p>
 744 <p>To learn how to use a particular module, use <code>perldoc Module::Name</code>.
 745 Typically you will want to <code>use Module::Name</code>, which will then give
 746 you access to exported functions or an OO interface to the module.</p>
 747 <p><a href="file://C|\msysgit\mingw\html/pod/perlfaq.html">the perlfaq manpage</a> contains questions and answers related to many common
 748 tasks, and often provides suggestions for good CPAN modules to use.</p>
 749 <p><a href="file://C|\msysgit\mingw\html/pod/perlmod.html">the perlmod manpage</a> describes Perl modules in general.  <a href="file://C|\msysgit\mingw\html/pod/perlmodlib.html">the perlmodlib manpage</a> lists the
 750 modules which came with your Perl installation.</p>
 751 <p>If you feel the urge to write Perl modules, <a href="file://C|\msysgit\mingw\html/pod/perlnewmod.html">the perlnewmod manpage</a> will give you
 752 good advice.</p>
 753 <p>
 754 </p>
 755 <hr />
 756 <h1><a name="author">AUTHOR</a></h1>
 757 <p>Kirrily ``Skud'' Robert &lt;<a href="mailto:skud@cpan.org">skud@cpan.org</a>&gt;</p>
 758 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 759 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 760 <big><strong><span class="block">&nbsp;perlintro -- a brief introduction and overview of Perl</span></strong></big>
 761 </td></tr>
 762 </table>
 763
 764 </body>
 765
 766 </html>