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Perl Cheat Sheet

Perl Cheat Sheet

Hitesh J

Perl stands for “Practical Extraction and Reporting Language” and is a high-level, general-purpose, interpreted, dynamic programming language developed by Larry Wall in 1987.

It was originally designed for text manipulation. But now, it is used for a wide range of tasks, including system administration, web development, network programming, GUI development, and more.

It supports HTML, XML, and other mark-up languages and supports third-party databases including Oracle, Postgres, MySQL, Sybase, etc. In addition, it is extensible, and there are 20,000+ third-party modules available from the Comprehensive Perl Archive Network.

A cheat sheet is a set of commands and notes that will be helpful for both beginners and professionals as a quick reference. This Perl cheat sheet will provide basic and advanced management and syntax of the Perl programming language.

Perl Functions

A Perl function is a group of statements used to perform a specific task. It allows users to divide their code into separate parts to reuse the code defined in the function.

Functions for Strings
Function Explanation
chop Remove the last character of a string and returns the character chopped.
chomp Removes any trailing string that corresponds to the current value of $/.
crypt It is a one-way hash function.
chr Returns the character represented by that NUMBER in the character set.
fc Returns the case folded version of EXPR. This is the internal function implementing the \F escape in double-quoted strings.
hex Interprets EXPR as a hex string and returns the corresponding numeric value.
index It searches for one string within another, but without the wildcard-like behavior of a full regular-expression pattern match.
lc Returns a lowercased version of EXPR.
lcfirst Returns the value of EXPR with the first character lowercased.
length Returns the length in characters of the value of EXPR.
oct Interprets EXPR as an octal string and returns the corresponding value.
ord Returns the numeric value of the first character of EXPR. If EXPR is an empty string, it returns 0.
pack Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE.
reverse Returns a list value consisting of the elements of LIST in the opposite order.
rindex Works just like index except that it returns the position of the last occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence beginning at or before that position.
sprintf Returns a string formatted by the usual printf conventions of the C library function sprintf.
substr Extracts a substring out of EXPR and returns it.
uc Returns an uppercased version of EXPR.
ucfirst Returns the value of EXPR with the first character in uppercase.
Numeric Functions
Function Explanation
abs Returns the absolute value of its argument.
atan2 Returns the arctangent of Y/X in the range -PI to PI.
cos Returns the cosine of EXPR.
exp Returns the (natural logarithm base) to the power of EXPR.
hex Interprets EXPR as a hex string and returns the corresponding numeric value.
int Returns the integer portion of EXPR.
log Returns the natural logarithm (base e) of EXPR.
oct Interprets EXPR as an octal string and returns the corresponding value.
rand Returns a random fractional number greater than or equal to 0 and less than the value of EXPR.
sin Returns the sine of EXPR (expressed in radians).
sqrt Return the positive square root of EXPR.
srand Sets and returns the random number seed for the rand operator.
Functions for real @ARRAYs
Function Explanation
shift Shifts the first value of the array off and returns it.
each When called on a hash in list context, returns a 2-element list consisting of the key and value for the next hash element.
keys Called in list context, returns a list consisting of all the keys of the named hash.
pop Pops and returns the last value of the array, shortening the array by one element.
push Treats ARRAY as a stack by appending the values of LIST to the end of ARRAY.
splice Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with LIST elements, if any. In the list context, returns the elements removed from the array.
unshift Does the opposite of a shift. Or the opposite of a push, depending on how you look at it.
values Returns a list consisting of all the values of the named hash.
delete Deletes the specified elements from that hash, so that exists on that element no longer returns true.
Functions for list data
Function Explanation
join Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns that new string.
map Evaluates the BLOCK or EXPR for each LIST element and composes a list of the results of each such evaluation.
reverse Returns a list value consisting of the elements of LIST in the opposite order.
sort Sorts the LIST and returns the sorted list value.
unpack Takes a string and expands it out into a list of values.
pack Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE.
read Read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE.
syscall Execute an arbitrary system call.
sysread Read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE.
sysseek Bypasses normal buffered IO, so mixing it with reads other than sysread.
syswrite Attempts to write LENGTH bytes from SCALAR to the file associated with FILEHANDLE.
Input and output functions
Function Explanation
binmode Arranges for FILEHANDLE to be read or written in “binary” or “text” mode on systems where the run-time libraries distinguish between binary and text files.
close Closes the file or pipe associated with the filehandle.
closedir Closes a directory opened by opendir and returns the success of that system call.
die Raise an exception.
fileno Returns the file descriptor for a filehandle or directory handle, or undefined if the filehandle is not open.
eof End of file.
format Declare a picture format for use by the write function.
getc Returns the next character from the input file attached to FILEHANDLE.
print Prints a string or a list of strings. Returns true if successful.
printf Output redirect to a filehandle.
read Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE.
Functions for filehandles, files, or directories
Function Explanation
chdir Change current working directory.
chmod Changes the permissions on a file/list of files.
chown Changes the owner (and group) of a list of files.
chroot Changes the root directory for the current process to dirname.
link Creates a new filename linked to the old filename.
lstat Stats a symbolic link instead of the file the symbolic link points to.
mkdir Create a directory
opendir Opens the directory EXPR, associating it with DIRHANDLE for processing, using the readdir function.
readlink Returns the value of a symbolic link, if symbolic links are implemented.
rename Changes the name of a file.
rmdir Remove a directory
select Returns the currently selected filehandle.
symlink Creates a new filename symbolically linked to the old filename.
umask Sets the umask for the process to EXPR and returns the previous value.
unlink Deletes a list of files.
Functions for fetching user and group info
Function Explanation
getlogin Return who is logged in at this TTY.
getpwent Get the next passwd record.
getpwnam Get passwd record given user login name.
getpwuid Get passwd record delivered user ID.
getgrgid GID Gets information by group ID.
getgrnam NAME Gets information by name.
getgrent Gets following group information.
gethostent Gets next host information.
Functions for fetching network info
Function Explanation
getnetbyname Get networks record given name.
getnetent Get the next network's record.
getpeername Find the other end of a socket connection.
getprotobyname Get protocol record given name.
getprotobynumber Get protocol record numeric protocol.
gethostbyaddr Gets information by IP address.
getprotoent Get the next protocols record.
gethostbyname Get host record given name.
accept Accepts a new socket.
bind Binds the NAME to the SOCKET.
connect Connects the NAME to the SOCKET.
getsockname Returns the name of the socket.
listen Starts listening on the specified SOCKET.
send Sends a message on the SOCKET.
shutdown Shuts down a SOCKET.

 

Comparison Operators

Comparison operators are used for comparing operands and return a Boolean value based upon whether the comparison is true or not. Perl has two types of comparison operator sets, numeric scalar values and string scalar values.

Boolean Operators
Operator Example
Logical AND operator && or and ($a && $b) is false
Logical OR operator || or or ($a or $b) is true
Logical NOT operator ! or not not($a) is false or !($a) is false
Arithmetic Operators
Numeric  String Description
Less than < lt
Greater than > gt
Less than or equal <= le
Greater than or equal >= ge
Equality == eq
Inequality != ne
Assoc Operators
Operator
Explanation
left -> Infix dereference operator
=++ Auto-increment
Auto-decrement
right ** Exponentiation
right \ Reference to an object (unary)
right ! ˜ Unary negation, bitwise complement
right + – Unary plus, minus
left Binds a scalar expression to a pattern match.
left Same, but negates the result.
left * / % x Multiplication, division, modulo, repetition.
left + – . Addition, subtraction, concatenation.
left >> << Bitwise shift right, bitwise shift left.
< > <= >= Numerical relational operators.
lt gt le ge String relational operators.
== != <=> Numerical equal, not equal, compare.
eq ne cmp Stringwise equal, not equal, compare.
left & Bitwise AND
left | ^ Bitwise OR, exclusive OR.
left && Logical AND
left || Logical OR
right = += -= *= Assignment operators
left , Comma operator

Regular Expressions

Regular Expressions are an essential part of Perl Programming used for searching the specified text pattern. The following cheat sheet contains the different classes, Characters, and modifiers used in the regular expression.

Character Classes
Classes Explanation
[abc.] Includes only one of specified characters i.e. ‘a’, ‘b’, ‘c’, or ‘.’
[a-j] Includes all the characters from a to j.
[a-z] Includes all lowercase characters from a to z.
[^az] Includes all characters except a and z.
\w Includes all characters like [a-z, A-Z, 0-9]
\d Matches for the digits like [0-9]
[ab][^cde] Matches that the characters a and b should not be followed by c, d, and e.
\s Matches for [\f\t\n\r] i.e form feed, tab, newline and carriage return.
\W Complement of \w
\D Complement of \d
\S Complement of \s
Anchors
Anchors Explanation
^ Matches at the beginning of the string.
$ Matches at the end of the string.
\b Matches at the word boundary of the string from \w to \W.
\A Matches at the beginning of the string.
\Z Matches at the ending of the string or before the newline.
\z Matches only at the end of the string.
\G Matches at the specified position pos().
\p{….} Unicode character class like IsLower, IsAlpha, etc.
\P{….} Complement of Unicode character class.
Meta Characters
Characters Explanation
. Any character except newline.
* Matches 0 or more times.
+ Matches 1 or more times.
? Matches 0 or more times.
() Used for grouping.
\ Use for quotes or special characters.
[] Used for a set of characters.
{} Used as repetition modifier.
Quantifiers
Quantifiers Explanation
a? Checks if ‘a’ occurs 0 or 1 time.
a+ Checks if ‘a’ occurs 1 or more times.
a* Checks if ‘a’ occurs 0 or more times.
a{2, 6} Checks if ‘a’ occurs 2 to 6 times.
a{2, } Checks if ‘a’ occurs 2 to infinite times.
a{2} Checks if ‘a’ occurs 2 times.
Modifiers
Modifiers Explanation
\g Used to replace all the occurrences of the string.
\gc Allows continued search after \g match fails.
\s Treats string as a single line.
i Turns off the case sensitivity.
\x Disregard all the white spaces.
(?#text) Used to add comments in the code.
(?:pattern) Used to match the pattern of the non-capturing group.
(?|pattern) Used to match the pattern of the branch test.
(?=pattern) Used for positive lookahead assertion.
(?!pattern) Used for negative lookahead assertion.
(<=pattern) Used for a positive look-behind assertion.
(<!pattern) Used for a negative look-behind assertion.
\t Used for inserting tab space.
\r Carriage return character.
\n Used for inserting a new line.
\h Used for inserting horizontal white space.
\v Used for inserting vertical white space.
\L Used for lowercase characters.
\U Used for upper case characters.

Variable and Special Variables

Variables are the reserved memory locations used to store and manipulate data throughout the program. When a variable is created, it occupies memory space. Perl provides three types of variables, scalars, lists, and hashes. They are used to manipulate the corresponding data types, including scalars, lists, and hashes.

Variables
Variables Explanation
$var Default variable
$var[10] 11st element of array @var
$p = \@var Now $p is a reference to @var
$$p[10] 11st element of array referenced by $p
$var[-1] Last element of array @var
$var[$x][$y] $y-th element of $x-th element of array @var
$var{’JAN’} A value from ‘hash’ %var
$p = \%var Now $p is a reference to hash %var
$$p{’JAN’} A value from hash referenced by $p
$#var Last index of array @var
@var The entire array
%var The entire hash
Special Variables
Variable Explanation
$ Default variable
$/ The input record separator, newline by default.
$\ The output record separator for the print operator.
$( The real GID (Group ID) of this process.
$) The effective GID (Group ID) of this process.
$& The last successful pattern match matches the string.
$< The real user ID of this process.
$> The effective user ID of this process.
$( The actual group ID of this process.
$) The influential group ID and groups of this process.
$~ The name of the current report format for the currently selected output channel.
$^ The name of the current top-of-page format for the currently selected output channel.
$^A The current value of the write() accumulator for format() lines.
$^L What formats output as a form feed. The default is \f.
$^T The time at which the program began running, in seconds since the epoch (beginning of 1970).
$^X The name used to execute the current copy of Perl.
$! Each element of %! has a true value only if $! is set to that value – %ERRNO.
$@ The Perl error from the last eval operator, i.e. the last exception that was caught.
$? The status returned by the last pipe close, backtick (“ ) command, successful call to wait() or waitpid(), or from the system() operator.
$. The current line number for the last filehandle accessed.
$% The current page number of the currently selected output channel.
$= The current page length (printable lines) of the currently selected output channel. The default is 60.
$- The number of lines left on the page of the currently selected output channel.
$| If set to nonzero, forces a flush right away and after every write or print on the currently selected output channel.
$0 Contains the name of the program being executed.
$+ The text matched by the highest used capture group of the last successful search pattern.

Command-line options

Perl comes with a wide range of command-line options that can be used to turn on or turn off different behaviors. You can create one-off command-line scripts to make your programs more concise.

Command Explanation
-a Turns on autosplit mode.
-c Checks syntax but does not execute.
-d Runs the script under the debugger.
-h Prints the Perl usage summary.
-m Imports the MODULE before executing the script.
-n Assumes an input loop around the script.
-P Runs the C preprocessor on the script before compilation by Perl.
-S Uses the PATH environment variable to search for the script.
-T Turns on taint checking.
-u Dumps core after compiling the script.
-U Allows Perl to perform unsafe operations.
-v Prints the version and patchlevel of your Perl executable.
-V Prints Perl configuration information.
-w Prints warnings about possible spelling errors.
-x Extracts the Perl program from the input stream.

 

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