Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular amongst programmers. Larry Wall continues to oversee development of the core language, and its upcoming version, Perl 6.
Perl borrows features from other programming languages including C, shell scripting (sh), AWK, and sed. The language provides powerful text processing facilities without the arbitrary data length limits of many contemporary Unix tools, facilitating easy manipulation of text files. It is also used for graphics programming, system administration, network programming, applications that require database access and CGI programming on the Web. Perl is nicknamed "the Swiss Army chainsaw of programming languages" due to its flexibility and adaptability.
Perl 2, released in 1988, featured a better regular expression engine. Perl 3, released in 1989, added support for binary data streams.
Originally the only documentation for Perl was a single (increasingly lengthy) man page. In 1991, Programming Perl (known to many Perl programmers as the "Camel Book" because of its cover) was published and became the de facto reference for the language. At the same time, the Perl version number was bumped to 4—not to mark a major change in the language but to identify the version that was documented by the book.
Perl 5.000 was released on October 17, 1994. It was a nearly complete rewrite of the interpreter, and it added many new features to the language, including objects, references, lexical (my) variables, and modules. Importantly, modules provided a mechanism for extending the language without modifying the interpreter. This allowed the core interpreter to stabilize, even as it enabled ordinary Perl programmers to add new language features. Perl 5 has been in active development since then.
Perl 5.001 was released on March 13, 1995. Perl 5.002 was released on February 29, 1996 with the new prototypes feature. This allowed module authors to make subroutines that behaved like Perl builtins. Perl 5.003 was released June 25, 1996, as a security release.
One of the most important events in Perl 5 history took place outside of the language proper and was a consequence of its module support. On October 26, 1995, the Comprehensive Perl Archive Network (CPAN) was established as a repository for Perl modules and Perl itself. At the time of writing, it carries over 20,000 modules by more than 8,000 authors. CPAN is widely regarded as one of the greatest strengths of Perl in practice.
Perl 5.004 was released on May 15, 1997, and included among other things the UNIVERSAL package, giving Perl a base object to which all classes were automatically derived and the ability to require versions of modules. In addition, Perl now supported running under Microsoft Windows and several other operating systems.
Perl 5.005 was released on July 22, 1998. This release included several enhancements to the Regex engine, new hooks into the backend through the B::* modules, the qr// regex quote operator, a large selection of other new core modules, and added support for several more operating systems, including BeOS.
In 2000, Larry Wall put forth a call for suggestions for a new version of Perl from the community. The process resulted in 361 RFCs (Request for Comments) documents which were to be used in guiding development of Perl 6. In 2001, work began on the apocalypses for Perl 6, a series of documents meant to summarize the change requests and present the design of the next generation of Perl. They were presented as a digest of the RFCs, rather than a formal document. At this point, Perl 6 existed only as a description of a language.
Perl 5.8 was first released on July 18, 2002, and had nearly yearly updates since then. The latest version of Perl 5.8 is 5.8.9, released December 14, 2008. Perl 5.8 improved unicode support, added a new IO implementation, added a new thread implementation, improved numeric accuracy, and added several new modules.
In 2004, work began on the Synopses – originally documents that summarized the Apocalypes, but which became the specification for the Perl 6 language. In February 2005, Audrey Tang began work on Pugs, a Perl 6 interpreter written in Haskell. This was the first real concerted effort towards making Perl 6 a reality. This effort stalled in 2006.
On December 18, 2007, the 20th anniversary of Perl 1.0, Perl 5.10.0 was released. Perl 5.10.0 included notable new features, which brought it closer to Perl 6. Some of these new features were a new switch statement (called "given"/"when"), regular expressions updates, and the smart match operator, "~~".
Around this same time, development began in earnest on another implementation of Perl 6 known as Rakudo Perl, developed in tandem with the Parrot virtual machine. As of November 2009, Rakudo Perl has had regular monthly releases and now is the most complete implementation of Perl 6.
A major change in the development process of Perl 5 occurred with Perl 5.11; the development community has switched to a monthly release cycle, with planned release dates three months ahead.
On April 12, 2010, Perl 5.12.0 was released. Notable core enhancements include new package NAME VERSION syntax, the Yada Yada operator (intended to mark placeholder code that is not yet implemented), implicit strictures, full Y2038 compliance, regexp conversion overloading, DTrace support, and Unicode 5.2. On September 7, 2010, Perl 5.12.2 was released; it contains updated modules and some documentation changes.
The latest development release of Perl 5 is 5.13.4, released by Florian Ragwitz on August 20, 2010.
When referring to the language, the name is normally capitalized (Perl) as a proper noun, as you would a spoken language (e.g. English or French). When referring to the interpreter program itself, the name is often uncapitalized (perl) because most Unix-like file systems are case-sensitive. Before the release of the first edition of Programming Perl, it was common to refer to the language as perl; Randal L. Schwartz, however, capitalized the language's name in the book to make it stand out better when typeset. This case distinction was subsequently documented as canonical.
There is some contention about the all-caps spelling "PERL," which the documentation declares incorrect The name is occasionally backronymed as Practical Extraction and Report Language (which appears at the top of the documentation and in some printed literature). Several backronyms have been suggested as equally canonical, including Wall's own humorous Pathologically Eclectic Rubbish Lister. Indeed, Wall claims that the name was intended to inspire many different expansions.
The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal). Its major features include support for multiple programming paradigms (procedural, object-oriented, and functional styles), reference counting memory management (without a cycle-detecting garbage collector), built-in support for text processing, and a large collection of third-party modules.
According to Larry Wall, Perl has two slogans. The first is "There's more than one way to do it", commonly known as TMTOWTDI. The second slogan is "Easy things should be easy and hard things should be possible".
Perl also takes features from shell programming. All variables are marked with leading sigils, which unambiguously identify the data type (for example, scalar, array, hash) of the variable in context. Importantly, sigils allow variables to be interpolated directly into strings. Perl has many built-in functions that provide tools often used in shell programming (although many of these tools are implemented by programs external to the shell) such as sorting, and calling on system facilities.
Perl takes lists from Lisp, associative arrays (hashes) from AWK, and regular expressions from sed. These simplify and facilitate many parsing, text-handling, and data-management tasks.
Perl 5 added features that support complex data structures, first-class functions (that is, closures as values), and an object-oriented programming model. These include references, packages, class-based method dispatch, and lexically scoped variables, along with compiler directives (for example, the strict pragma). A major additional feature introduced with Perl 5 was the ability to package code as reusable modules. Larry Wall later stated that "The whole intent of Perl 5's module system was to encourage the growth of Perl culture rather than the Perl core."
All versions of Perl do automatic data-typing and automatic memory-management. The interpreter knows the type and storage requirements of every data object in the program; it allocates and frees storage for them as necessary using reference counting (so it cannot deallocate circular data structures without manual intervention). Legal type-conversions — for example, conversions from number to string — are done automatically at run time; illegal type conversions are fatal errors.
Perl has many features that ease the programmer's task at the expense of greater CPU and memory requirements. These include automatic memory management; dynamic typing; strings, lists, and hashes; regular expressions; introspection; and an eval() function.
Wall was trained as a linguist, and the design of Perl is very much informed by linguistic principles. Examples include Huffman coding (common constructions should be short), good end-weighting (the important information should come first), and a large collection of language primitives. Perl favors language constructs that are concise and natural for humans to read and write, even where they complicate the Perl interpreter.
Perl syntax reflects the idea that "things that are different should look different". For example, scalars, arrays, and hashes have different leading sigils. Array indices and hash keys use different kinds of braces. Strings and regular expressions have different standard delimiters. This approach can be contrasted with languages such as Lisp, where the same S-expression construct and basic syntax are used for many different purposes.
Perl does not enforce any particular programming paradigm (procedural, object-oriented, functional, or others) or even require the programmer to choose among them.
There is a broad practical bent to both the Perl language and the community and culture that surround it. The preface to Programming Perl begins: "Perl is a language for getting your job done." One consequence of this is that Perl is not a tidy language. It includes many features, tolerates exceptions to its rules, and employs heuristics to resolve syntactical ambiguities. Because of the forgiving nature of the compiler, bugs can sometimes be hard to find. Discussing the variant behaviour of built-in functions in list and scalar contexts, the perlfunc(1) manual page says: "In general, they do what you want, unless you want consistency."
In addition to Larry Wall's two slogans mentioned above, Perl has several mottos that convey aspects of its design and use, including "Perl: the Swiss Army Chainsaw of Programming Languages" and "No unnecessary limits". Perl has also been called "The Duct Tape of the Internet".
No written specification or standard for the Perl language exists for Perl versions through Perl 5, and there are no plans to create one for the current version of Perl. There has been only one implementation of the interpreter, and the language has evolved along with it. That interpreter, together with its functional tests, stands as a de facto specification of the language. Perl 6, however, started with a specification, and several projects aim to implement some or all of the specification.
Ever since the early days of the Web, programmers have used Perl to write CGI scripts. Perl is known as one of "the three Ps" (along with Python and PHP), the most popular dynamic languages for writing Web applications. It is also an integral component of the popular LAMP solution stack for web development. Large projects written in Perl include cPanel, Slash, Bugzilla, RT, TWiki, and Movable Type. Many high-traffic websites use Perl extensively. Examples include Amazon.com, bbc.co.uk, Priceline.com, Craigslist, IMDb, LiveJournal, Slashdot and Ticketmaster.
Perl is often used as a glue language, tying together systems and interfaces that were not specifically designed to interoperate, and for "data munging", that is, converting or processing large amounts of data for tasks such as creating reports. In fact, these strengths are intimately linked. The combination makes Perl a popular all-purpose language for system administrators, particularly because short programs can be entered and run on a single command line.
With a degree of care, Perl code can be made portable across Windows and Unix. Portable Perl code is often used by suppliers of software (both COTS and bespoke) to simplify packaging and maintenance of software build- and deployment-scripts.
Graphical user interfaces (GUIs) may be developed using Perl. For example, Perl/Tk is commonly used to enable user interaction with Perl scripts. Such interaction may be synchronous or asynchronous, using callbacks to update the GUI. For more information about the technologies involved, see Tk, Tcl and WxPerl.
Perl is also widely used in finance and in bioinformatics, where it is valued for rapid application development and deployment and for its capability to handle large data-sets.
The interpreter has an object-oriented architecture. All of the elements of the Perl language—scalars, arrays, hashes, coderefs, file handles—are represented in the interpreter by C structs. Operations on these structs are defined by a large collection of macros, typedefs, and functions; these constitute the Perl C API. The Perl API can be bewildering to the uninitiated, but its entry points follow a consistent naming-scheme, which provides guidance to those who use it.
The life of a Perl interpreter divides broadly into a compile phase and a run phase. In Perl, the phases are the major stages in the interpreter's life-cycle. Each interpreter goes through each phase only once, and the phases follow in a fixed sequence.
Most of what happens in Perl's compile phase is compilation, and most of what happens in Perl's run phase is execution, but there are significant exceptions. Perl makes important use of its capability to execute Perl code during the compile phase. Perl will also delay compilation into the run phase. The terms that indicate the kind of processing that is actually occurring at any moment are compile time and run time. Perl is in compile time at most points during the compile phase, but compile time may also be entered during the run phase. The compile time for code in a string argument passed to the eval built-in occurs during the run phase. Perl is often in run time during the compile phase and spends most of the run phase in run time. Code in BEGIN blocks executes at run time but in the compile phase.
At compile time, the interpreter parses Perl code into a syntax tree. At run time, it executes the program by walking the tree. Text is parsed only once, and the syntax tree is subject to optimization before it is executed, so that execution is relatively efficient. Compile-time optimizations on the syntax tree include constant folding and context propagation, but peephole optimization is also performed.
Perl has a Turing-complete grammar because parsing can be affected by run-time code executed during the compile phase. Therefore, Perl cannot be parsed by a straight Lex/Yacc lexer/parser combination. Instead, the interpreter implements its own lexer, which coordinates with a modified GNU bison parser to resolve ambiguities in the language.
It is often said that "Only perl can parse Perl," meaning that only the Perl interpreter (perl) can parse the Perl language (Perl), but even this is not, in general, true. Because the Perl interpreter can simulate a Turing machine during its compile phase, it would need to decide the Halting Problem in order to complete parsing in every case. It's a long-standing result that the Halting Problem is undecidable, and therefore not even perl can always parse Perl. Perl makes the unusual choice of giving the user access to its full programming power in its own compile phase. The cost in terms of theoretical purity is high, but practical inconvenience seems to be rare.
Other programs that undertake to parse Perl, such as source-code analyzers and auto-indenters, have to contend not only with ambiguous syntactic constructs but also with the undecidability of Perl parsing in the general case. Adam Kennedy's PPI project focused on parsing Perl code as a document (retaining its integrity as a document), instead of parsing Perl as executable code (which not even Perl itself can always do). It was Kennedy who first conjectured that, "parsing Perl suffers from the 'Halting Problem'." and this was later proved.
Perl is distributed with some 120,000 functional tests. These run as part of the normal build process and extensively exercise the interpreter and its core modules. Perl developers rely on the functional tests to ensure that changes to the interpreter do not introduce bugs; conversely, Perl users who see that the interpreter passes its functional tests on their system can have a high degree of confidence that it is working properly.
Maintenance of the Perl interpreter has become increasingly difficult over the years. The code base has undergone continuous development since 1994. The code has been optimized for performance at the expense of simplicity, clarity, and strong internal interfaces. New features have been added, yet virtually complete backward compatibility with earlier versions is maintained. Major releases of Perl were coordinated by Perl pumpkings, which handled integrating patch submissions and bug fixes, but the language has since changed to a rotating, monthly release cycle. Development discussion takes place via the perl5_porters mailing list. As of Perl 5.11, development efforts have included refactoring certain core modules known as 'dual lifed' modules out of the Perl core to help alleviate some of these problems.
Because of unusual changes required for the Mac OS Classic environment, a special port called MacPerl was shipped independently.
The Comprehensive Perl Archive Network (CPAN) carries a complete list of supported platforms with links to the distributions available on each. CPAN is also the source for publicly available Perl modules that are not part of the core Perl distribution.
ActivePerl is an open source distribution from ActiveState that has regular releases that track the core Perl releases. This free distribution also includes the Perl Package Manager (PPM), a popular tool for installing, removing, upgrading, and managing the use of common Perl modules.
Strawberry Perl is an open source distribution for Windows. It has had regular, quarterly releases since January 2008, including new modules as feedback and requests come in. Strawberry Perl aims to be able to install modules like standard Perl distributions on other platforms, including compiling XS modules.
A community project was launched by Adam Kennedy on behalf of The Perl Foundation in June 2006. A community website for "all things Windows and Perl." A major aim of this project is to provide production-quality alternative Perl distributions that include an embedded C compiler and build tools, so as to enable Windows users to install modules directly from the CPAN. A related version with research and experimental work was done in the Vanilla Perl distribution.
The Cygwin emulation layer is another popular way of running Perl under Windows. Cygwin provides a Unix-like environment on Windows, and both perl and cpan are conveniently available as standard pre-compiled packages in the Cygwin setup program. Because Cygwin also includes the gcc, compiling Perl from source is also possible.
n character whose interpretation is altered by the preceding escape character (a backslash).
The canonical form of the program is slightly more verbose:
The hash mark character introduces a comment in Perl, which runs up to the end of the line of code and is ignored by the compiler. The comment used here is of a special kind: it’s called the shebang line. This tells Unix-like operating systems where to find the Perl interpreter, making it possible to invoke the program without explicitly mentioning perl. (Note that, on Microsoft Windows systems, Perl programs are typically invoked by associating the .pl with the Perl interpreter. In order to deal with such circumstances, perl detects the shebang line and parses it for switches; therefore, it is not strictly true that the shebang line is ignored by the compiler.)
The second line in the canonical form includes a semicolon, which is used to separate statements in Perl. With only a single statement in a block or file, a separator is unnecessary, so it can be omitted from the minimal form of the program—or more generally from the final statement in any block or file. The canonical form includes it because it is common to terminate every statement even when it is unnecessary to do so, as this makes editing easier: code can be added to, or moved away from, the end of a block or file without having to adjust semicolons.
Version 5.10 of Perl introduces a say function that implicitly appends a newline character to its output, making the minimal "Hello world" program even shorter:
{| class="wikitable" |- ! Type ! Sigil ! Example ! Description |- |Scalar |$ |$foo |a single value; it may be a number, a string, a file handle, or a reference. |- |Array |@ |@foo |An ordered collection of scalars. |- |Associative array |% |%foo |A map from strings to scalars; the strings are called keys, and the scalars are called values. Also known as a hash. |- | |none |$foo or FOO |A map to a file, device, pipe, or scalar that is open for reading, writing, or both. |- |Subroutine |& |&foo; |A piece of code that may be passed arguments, be executed, and return data. |- |Typeglob |* |*foo |The symbol table entry for all types with the name 'foo'. |}
To include a double quotation mark in a string, precede it with a backslash or enclose the string in single quotes. To include a single quotation mark, precede it with a backslash or enclose the string in double quotes. Strings can also be quoted with the q and qq quote-like operators. 'this' is identical to q(this) and "$this" is identical to qq($this).
Finally, multiline strings can be defined using here documents:
Numbers (numeric constants) do not require quotation. Perl will convert numbers into strings and vice versa depending on the context in which they are used. When strings are converted into numbers, trailing non-numeric parts of the strings are discarded. If no leading part of a string is numeric, the string will be converted to the number 0. In the following example, the strings $n and $m are treated as numbers. This code prints the number '5'. The values of the variables remain the same. Note that in Perl, + is always the numeric addition operator. The string concatenation operator is the period.
int chops off the fractional part, rounding towards zero; POSIX::ceil and POSIX::floor round always up and always down, respectively. The number-to-string conversion of printf "%f" or sprintf "%f" round out even, use bankers' rounding.
Perl also has a boolean context that it uses in evaluating conditional statements. The following values all evaluate as false in Perl:
All other (non-zero evaluating) values evaluate to true. This includes the odd self-describing literal string of "0 but true", which in fact is 0 as a number, but true when used as a boolean. All non-numeric strings also have this property, but this particular string is truncated by Perl without a numeric warning. A less explicit but more conceptually portable version of this string is '0E0' or '0e0', which does not rely on characters being evaluated as 0, because '0E0' is literally zero times ten to the power zero.
Evaluated boolean expressions are also scalar values. The documentation does not promise which particular value of true or false is returned. Many boolean operators return 1 for true and the empty-string for false. The defined() function determines whether a variable has any value set. In the above examples, defined($false) is true for every value except undef.
If either 1 or 0 are specifically needed, an explicit conversion can be done using the conditional operator:
The qw() quote-like operator allows the definition of a list of strings without typing of quotes and commas. Almost any delimiter can be used instead of parentheses. The following lines are equivalent:
The split function returns a list of strings, which are split from a string expression using a delimiter string or regular expression.
Individual elements of a list are accessed by providing a numerical index in square brackets. The scalar sigil must be used. Sublists (array slices) can also be specified, using a range or list of numeric indices in brackets. The array sigil is used in this case. For example, $month[3] is "April" (the first element in an array has an index value of 0), and @month[4..6] is ("May", "June", "July").
=> operator (sometimes called a fat comma), rather than a comma, they may be unquoted (barewords). The following lines are equivalent:
Individual values in a hash are accessed by providing the corresponding key, in curly braces. The $ sigil identifies the accessed element as a scalar. For example, $favorite{joe} equals 'red'. A hash can also be initialized by setting its values individually:
Multiple elements may be accessed using the @ sigil instead (identifying the result as a list). For example,
@favorite{'joe', 'sam'} equals ('red', 'blue').
Originally, File Handles could only be created with package variables, using the ALL_CAPS convention to distinguish it from other variables. Perl 5.6 and newer also accept a scalar variable, which will be set (autovivified) to a reference to an anonymous file handle, in place of a named file handle. Using the ALL_CAPS method for file handles is considered deprecated by the community.
$# sigil. The latter gives the index of the last element in the array, not the number of elements. The expressions scalar(@array) and ($#array + 1) are equivalent.
# Similar to the above, but sorted alphabetically foreach my $next_name (sort keys %addressbook) { print "$next_name lives at $addressbook{$next_name}\n"; }
Perl has several kinds of control structures.
It has block-oriented control structures, similar to those in the C, Javascript, and Java programming languages. Conditions are surrounded by parentheses, and controlled blocks are surrounded by braces:
label while ( cond ) { ... } label while ( cond ) { ... } continue { ... } label for ( init-expr ; cond-expr ; incr-expr ) { ... } label foreach var ( list ) { ... } label foreach var ( list ) { ... } continue { ... } if ( cond ) { ... } if ( cond ) { ... } else { ... } if ( cond ) { ... } elsif ( cond ) { ... } else { ... }
Where only a single statement is being controlled, statement modifiers provide a more-concise syntax:
statement if cond ; statement unless cond ; statement while cond ; statement until cond ; statement foreach list ;
Short-circuit logical operators are commonly used to affect control flow at the expression level:
expr and expr
expr && expr
expr or expr
expr
(The "and" and "or" operators are similar to && and
The flow control keywords next (corresponding to C's continue), last (corresponding to C's break), return, and redo are expressions, so they can be used with short-circuit operators.
Perl also has two implicit looping constructs, each of which has two forms:
results = grep { ... } list results = grep expr, list results = map { ... } list results = map expr, list
grep returns all elements of list for which the controlled block or expression evaluates to true. map evaluates the controlled block or expression for each element of list and returns a list of the resulting values. These constructs enable a simple functional programming style.
Up until the 5.10.0 release, there was no switch statement in Perl 5. From 5.10.0 onward, a multi-way branch statement called given/when is available, which takes the following form:
use v5.10; # must be present to import the new 5.10 functions given ( expr ) { when ( cond ) { ... } default { ... } }
Syntactically, this structure behaves similarly to switch statements found in other languages, but with a few important differences. The largest is that unlike switch/case structures, given/when statements break execution after the first successful branch, rather than waiting for explicitly defined break commands. Conversely, explicit continues are instead necessary to emulate switch behavior.
For those not using Perl 5.10, the Perl documentation describes a half-dozen ways to achieve the same effect by using other control structures. There is also a Switch module, which provides functionality modeled on the forthcoming Perl 6 re-design. It is implemented using a source filter, so its use is unofficially discouraged.
Perl includes a goto label statement, but it is rarely used. Situations where a goto is called for in other languages don't occur as often in Perl because of its breadth of flow control options.
There is also a goto ⊂ statement that performs a tail call. It terminates the current subroutine and immediately calls the specified sub. This is used in situations where a caller can perform more-efficient stack management than Perl itself (typically because no change to the current stack is required), and in deep recursion, tail calling can have substantial positive impact on performance because it avoids the overhead of scope/stack management on return.
sub keyword and are invoked simply by naming them. If the subroutine in question has not yet been declared, invocation requires either parentheses after the function name or an ampersand (&) before it. But using & without parentheses will also implicitly pass the arguments of the current subroutine to the one called, and using & with parentheses will bypass prototypes.
# Parentheses are required here if the subroutine is defined later in the code foo(); &foo; # (this also works, but has other consequences regarding arguments passed to the subroutine)
# Defining a subroutine sub foo { ... }
foo; # Here parentheses are not required
A list of arguments may be provided after the subroutine name. Arguments may be scalars, lists, or hashes.
Arrays are expanded to their elements; hashes are expanded to a list of key/value pairs; and the whole lot is passed into the subroutine as one flat list of scalars.
Whatever arguments are passed are available to the subroutine in the special array @_. The elements of @_ are references to the actual arguments; changing an element of @_ changes the corresponding argument.
Elements of @_ may be accessed by subscripting it in the usual way.
However, the resulting code can be difficult to read, and the parameters have pass-by-reference semantics, which may be undesirable.
One common idiom is to assign @_ to a list of named variables.
This provides mnemonic parameter names and implements pass-by-value semantics. The my keyword indicates that the following variables are lexically scoped to the containing block.
Another idiom is to shift parameters off of @_. This is especially common when the subroutine takes only one argument or for handling the $self argument in object-oriented modules.
Subroutines may assign @_ to a hash to simulate named arguments; this is recommended in Perl Best Practices for subroutines that are likely to ever have more than three parameters.
Subroutines may return values.
If the subroutine does not exit via a return statement, then it returns the last expression evaluated within the subroutine body. Arrays and hashes in the return value are expanded to lists of scalars, just as they are for arguments.
The returned expression is evaluated in the calling context of the subroutine; this can surprise the unwary.
$x = list; # returns 6 - last element of list $x = array; # returns 3 - number of elements in list @x = list; # returns (4, 5, 6) @x = array; # returns (4, 5, 6)
A subroutine can discover its calling context with the wantarray function.
$x = either; # returns "Oranges" @x = either; # returns (1, 2)
The Perl regular-expression syntax was originally taken from Unix Version 8 regular expressions. However, it diverged before the first release of Perl and has since grown to include far more features. Many other languages and applications are now adopting Perl compatible regular expressions over POSIX regular expressions, such as PHP, Ruby, Java, Microsoft's .NET Framework, and the Apache HTTP server.
Regular-expression syntax is extremely compact, owing to history. The first regular-expression dialects were only slightly more expressive than globs, and the syntax was designed so that an expression would resemble the text that it matches. This meant using no more than a single punctuation character or a pair of delimiting characters to express the few supported assertions. Over time, the expressiveness of regular expressions grew tremendously, but the syntax design was never revised and continues to rely on punctuation. As a result, regular expressions can be cryptic and extremely dense.
m// (match) operator introduces a regular-expression match. (If it is delimited by slashes, as in all of the examples here, then the leading m may be omitted for brevity. If the m is present, as in all of the following examples, other delimiters can be used in place of slashes.) In the simplest case, an expression such as
evaluates to true if and only if the string $x matches the regular expression abc.
The s/// (substitute) operator, on the other hand, specifies a search-and-replace operation:
Another use of regular expressions is to specify delimiters for the split function:
The split function creates a list of the parts of the string that are separated by matches of the regular expression. In this example, a line is divided into a list of its comma-separated parts, and this list is then assigned to the @words array.
Because the compact syntax of regular expressions can make them dense and cryptic, the /x modifier was added in Perl to help programmers write more-legible regular expressions. It allows programmers to place whitespace and comments inside regular expressions:
$1, $2, $3, ..., and a list of captured strings is returned as the value of the match.
Captured strings $1, $2, $3, ... can be used later in the code.
Perl regular expressions also allow built-in or user-defined functions apply to the captured match, by using the /e modifier:
sub Point::distance { my ($self, $from) = @_; my ($dx, $dy) = ($$self[0] - $$from[0], $$self[1] - $$from[1]); sqrt($dx * $dx + $dy * $dy); }
This class can be used by invoking new() to construct instances, and invoking distance on those instances.
Many modern Perl applications use the Moose object system. Moose is built on top of Class::MOP, a meta-object protocol, providing complete introspection for all Moose-using classes. Thus you can ask classes about their attributes, parents, children, methods, etc. using a simple API.
Moose classes:
Moose roles:
class Point3D extends Point { has 'z' => (isa => 'Num', is => 'rw');
after clear { $self->z(0); } method set_to (Num $x, Num $y, Num $z) { $self->x($x); $self->y($y); $self->z($z); } }
This is a class named Point3D that extends another class named Point explained in Moose examples. Id adds to its base class a new attribute z, redefines the method set_to and extends the method clear.
In early versions of Perl, database interfaces were created by relinking the interpreter with a client-side database library. This was sufficiently difficult that it was done for only a few of the most-important and most widely used databases, and it restricted the resulting perl executable to using just one database interface at a time.
In Perl 5, database interfaces are implemented by Perl DBI modules. The DBI (Database Interface) module presents a single, database-independent interface to Perl applications, while the DBD (Database Driver) modules handle the details of accessing some 50 different databases; there are DBD drivers for most ANSI SQL databases.
DBI provides caching for database handles and queries, which can greatly improve performance in long-lived execution environments such as mod_perl, helping high-volume systems avert load spikes as in the Slashdot effect.
In modern Perl applications, especially those written using Web application frameworks such as Catalyst, the DBI module is often used indirectly via object-relational mappers such as , or which generate SQL queries and handle data transparently to the application author.
Large Perl programs start slower than similar programs in compiled languages because perl has to compile the source every time it runs. In a talk at the YAPC::Europe 2005 conference and subsequent article "A Timely Start," Jean-Louis Leroy found that his Perl programs took much longer to run than he expected because the perl interpreter spent much of the time finding modules because of his over-large include path. Unlike Java, Python, and Ruby, Perl has only experimental support for pre-compiling. Therefore Perl programs pay this overhead penalty on every execution. The run phase of typical programs is long enough that amortized startup time is not substantial, but results in benchmarks that measure very short execution times are likely to be skewed.
A number of tools have been introduced to improve this situation. The first such tool was Apache's mod_perl, which sought to address one of the most-common reasons that small Perl programs were invoked rapidly: CGI Web development. ActivePerl, via Microsoft ISAPI, provides similar performance improvements.
Once Perl code is compiled, there is additional overhead during the execution phase that typically isn't present for programs written in compiled languages such as C or C++. Examples of such overhead include bytecode interpretation, reference-counting memory management, and dynamic type checking.
In such situations, the most-critical routines of a Perl program can be written in other languages such as C or Assembler, which can be connected to Perl via simple Inline modules or the more-complex-but-flexible XS mechanism. Nicholas Clark, a Perl core developer, discusses some Perl design trade-offs and some solutions in When perl is not quite fast enough.
In extreme cases, optimizing Perl can require intimate knowledge of the interpreter's workings rather than skill with algorithms, the Perl language, or general principles of optimization.
Larry Wall spent the next few years digesting the RFCs and synthesizing them into a coherent framework for Perl 6. He has presented his design for Perl 6 in a series of documents called "apocalypses," which are numbered to correspond to chapters in Programming Perl ("The Camel Book"). The current, not-yet-finalized specification of Perl 6 is encapsulated in design documents called Synopses, which are numbered to correspond to Apocalypses.
Perl 6 is not intended to be backward compatible, although there will be a compatibility mode.
Thesis work by Bradley M. Kuhn, overseen by Larry Wall, considered the possible use of the Java virtual machine as a runtime for Perl. Kuhn's thesis showed this approach to be problematic, and in 2001, it was decided that Perl 6 would run on a cross-language virtual machine called Parrot. This will mean that other languages targeting the Parrot will gain native access to CPAN, allowing some level of cross-language development.
In 2005, Audrey Tang created the pugs project, an implementation of Perl 6 in Haskell. This was, and continues to act as, a test platform for the Perl 6 language (separate from the development of the actual implementation) allowing the language designers to explore. The pugs project spawned an active Perl/Haskell cross-language community centered around the freenode #perl6 irc channel.
A number of features in the Perl 6 language now show similarities to Haskell.
As of early 2009, Perl 6 development is primarily centred around Rakudo Perl 6, an implementation running on top of the Parrot virtual machine. Another implementation, Mildew, is also under active development and does not use Parrot.
Development of Perl 5 is also continuing. Perl 5.12.0 was released in April 2010, with some new features influenced by the design of Perl 6. New maintenance versions are scheduled to be released on a monthly basis, the latest one, 5.12.2, was released on September 7, 2010.
Each talk is first given at various Perl conferences and is eventually also published online.
In the parlance of Perl culture, Perl programmers are known as Perl hackers, and from this derives the practice of writing short programs to print out the phrase "Just another Perl hacker",. In the spirit of the original concept, these programs are moderately obfuscated and short enough to fit into the signature of an email or Usenet message. The "canonical" JAPH includes the comma at the end, although this is often omitted.
A Perl Golf Apocalypse was held at Perl Conference 4.0 in Monterey, California in July 2000.
Lingua::Romana::Perligata, for example, allows writing programs in Latin. Upon execution of such a program, the module translates its source code into regular Perl and runs it.
The Perl community has set aside the "Acme" namespace for modules that are fun in nature (but its scope has widened to include exploratory or experimental code or any other module that is not meant to ever be used in production). Some of the Acme modules are deliberately implemented in amusing ways. This includes Acme::Bleach, one of the first modules in the Acme:: namespace, which allows the program's source code to be "whitened" (i.e., all characters replaced with whitespace) and yet still work.
* Comparison of programming languages
* Category:Dynamic programming languages Category:Dynamically typed programming languages Category:Free compilers and interpreters Category:Free software programmed in C Category:Procedural programming languages Category:Object-oriented programming languages Category:Scripting languages Category:Text-oriented programming languages Category:Unix programming tools Category:Cross-platform software Category:American inventions
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