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PERLTRAP(1)	       Perl Programmers Reference Guide		   PERLTRAP(1)

       perltrap - Perl traps for the unwary

       The biggest trap of all is forgetting to "use warnings" or use the -w
       switch; see perllexwarn and perlrun. The second biggest trap is not
       making your entire program runnable under "use strict".	The third
       biggest trap is not reading the list of changes in this version of
       Perl; see perldelta.

   Awk Traps
       Accustomed awk users should take special note of the following:

       ·   A Perl program executes only once, not once for each input line.
	   You can do an implicit loop with "-n" or "-p".

       ·   The English module, loaded via

	       use English;

	   allows you to refer to special variables (like $/) with names (like
	   $RS), as though they were in awk; see perlvar for details.

       ·   Semicolons are required after all simple statements in Perl (except
	   at the end of a block).  Newline is not a statement delimiter.

       ·   Curly brackets are required on "if"s and "while"s.

       ·   Variables begin with "$", "@" or "%" in Perl.

       ·   Arrays index from 0.	 Likewise string positions in substr() and

       ·   You have to decide whether your array has numeric or string

       ·   Hash values do not spring into existence upon mere reference.

       ·   You have to decide whether you want to use string or numeric

       ·   Reading an input line does not split it for you.  You get to split
	   it to an array yourself.  And the split() operator has different
	   arguments than awk's.

       ·   The current input line is normally in $_, not $0.  It generally
	   does not have the newline stripped.	($0 is the name of the program
	   executed.)  See perlvar.

       ·   $<digit> does not refer to fields--it refers to substrings matched
	   by the last match pattern.

       ·   The print() statement does not add field and record separators
	   unless you set $, and "$\".	You can set $OFS and $ORS if you're
	   using the English module.

       ·   You must open your files before you print to them.

       ·   The range operator is "..", not comma.  The comma operator works as
	   in C.

       ·   The match operator is "=~", not "~".	 ("~" is the one's complement
	   operator, as in C.)

       ·   The exponentiation operator is "**", not "^".  "^" is the XOR
	   operator, as in C.  (You know, one could get the feeling that awk
	   is basically incompatible with C.)

       ·   The concatenation operator is ".", not the null string.  (Using the
	   null string would render "/pat/ /pat/" unparsable, because the
	   third slash would be interpreted as a division operator--the
	   tokenizer is in fact slightly context sensitive for operators like
	   "/", "?", and ">".  And in fact, "." itself can be the beginning of
	   a number.)

       ·   The "next", "exit", and "continue" keywords work differently.

       ·   The following variables work differently:

		 Awk	   Perl
		 ARGC	   scalar @ARGV (compare with $#ARGV)
		 ARGV[0]   $0
		 FNR	   $. - something
		 FS	   (whatever you like)
		 NF	   $#Fld, or some such
		 NR	   $.
		 OFMT	   $#
		 OFS	   $,
		 ORS	   $\
		 RLENGTH   length($&)
		 RS	   $/
		 RSTART	   length($`)
		 SUBSEP	   $;

       ·   You cannot set $RS to a pattern, only a string.

       ·   When in doubt, run the awk construct through a2p and see what it
	   gives you.

   C/C++ Traps
       Cerebral C and C++ programmers should take note of the following:

       ·   Curly brackets are required on "if"'s and "while"'s.

       ·   You must use "elsif" rather than "else if".

       ·   The "break" and "continue" keywords from C become in Perl "last"
	   and "next", respectively.  Unlike in C, these do not work within a
	   "do { } while" construct.  See "Loop Control" in perlsyn.

       ·   The switch statement is called "given/when" and only available in
	   perl 5.10 or newer.	See "Switch Statements" in perlsyn.

       ·   Variables begin with "$", "@" or "%" in Perl.

       ·   Comments begin with "#", not "/*" or "//".  Perl may interpret
	   C/C++ comments as division operators, unterminated regular
	   expressions or the defined-or operator.

       ·   You can't take the address of anything, although a similar operator
	   in Perl is the backslash, which creates a reference.

       ·   "ARGV" must be capitalized.	$ARGV[0] is C's "argv[1]", and
	   "argv[0]" ends up in $0.

       ·   System calls such as link(), unlink(), rename(), etc. return
	   nonzero for success, not 0. (system(), however, returns zero for

       ·   Signal handlers deal with signal names, not numbers.	 Use "kill -l"
	   to find their names on your system.

   Sed Traps
       Seasoned sed programmers should take note of the following:

       ·   A Perl program executes only once, not once for each input line.
	   You can do an implicit loop with "-n" or "-p".

       ·   Backreferences in substitutions use "$" rather than "\".

       ·   The pattern matching metacharacters "(", ")", and "|" do not have
	   backslashes in front.

       ·   The range operator is "...", rather than comma.

   Shell Traps
       Sharp shell programmers should take note of the following:

       ·   The backtick operator does variable interpolation without regard to
	   the presence of single quotes in the command.

       ·   The backtick operator does no translation of the return value,
	   unlike csh.

       ·   Shells (especially csh) do several levels of substitution on each
	   command line.  Perl does substitution in only certain constructs
	   such as double quotes, backticks, angle brackets, and search

       ·   Shells interpret scripts a little bit at a time.  Perl compiles the
	   entire program before executing it (except for "BEGIN" blocks,
	   which execute at compile time).

       ·   The arguments are available via @ARGV, not $1, $2, etc.

       ·   The environment is not automatically made available as separate
	   scalar variables.

       ·   The shell's "test" uses "=", "!=", "<" etc for string comparisons
	   and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the
	   reverse of Perl, which uses "eq", "ne", "lt" for string
	   comparisons, and "==", "!=" "<" etc for numeric comparisons.

   Perl Traps
       Practicing Perl Programmers should take note of the following:

       ·   Remember that many operations behave differently in a list context
	   than they do in a scalar one.  See perldata for details.

       ·   Avoid barewords if you can, especially all lowercase ones.  You
	   can't tell by just looking at it whether a bareword is a function
	   or a string.	 By using quotes on strings and parentheses on
	   function calls, you won't ever get them confused.

       ·   You cannot discern from mere inspection which builtins are unary
	   operators (like chop() and chdir()) and which are list operators
	   (like print() and unlink()).	 (Unless prototyped, user-defined
	   subroutines can only be list operators, never unary ones.)  See
	   perlop and perlsub.

       ·   People have a hard time remembering that some functions default to
	   $_, or @ARGV, or whatever, but that others which you might expect
	   to do not.

       ·   The <FH> construct is not the name of the filehandle, it is a
	   readline operation on that handle.  The data read is assigned to $_
	   only if the file read is the sole condition in a while loop:

	       while (<FH>)	 { }
	       while (defined($_ = <FH>)) { }..
	       <FH>;  # data discarded!

       ·   Remember not to use "=" when you need "=~"; these two constructs
	   are quite different:

	       $x =  /foo/;
	       $x =~ /foo/;

       ·   The "do {}" construct isn't a real loop that you can use loop
	   control on.

       ·   Use "my()" for local variables whenever you can get away with it
	   (but see perlform for where you can't).  Using "local()" actually
	   gives a local value to a global variable, which leaves you open to
	   unforeseen side-effects of dynamic scoping.

       ·   If you localize an exported variable in a module, its exported
	   value will not change.  The local name becomes an alias to a new
	   value but the external name is still an alias for the original.

       As always, if any of these are ever officially declared as bugs,
       they'll be fixed and removed.

perl v5.18.2			  2014-01-06			   PERLTRAP(1)

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