perltrap

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 index().
• You have to decide whether your array has numeric or string indices.
• Hash values do not spring into existence upon mere reference.
• You have to decide whether you want to use string or numeric comparisons.
• 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
        FILENAME  $ARGV
        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 success.)
• Signal handlers deal with signal names, not numbers. Use "kill -l" to find their names on your system.

JavaScript Traps

Judicious JavaScript programmers should take note of the following:

• In Perl, binary "+" is always addition. "$string1 + $string2" converts both strings to numbers and then adds them. To concatenate two strings, use the "." operator.
• The "+" unary operator doesn't do anything in Perl. It exists to avoid syntactic ambiguities.
• Unlike "for...in", Perl's "for" (also spelled "foreach") does not allow the left-hand side to be an arbitrary expression. It must be a variable:

     for my $variable (keys %hash) {
          ...
     }
      

  Furthermore, don't forget the "keys" in there, as "foreach my $kv (%hash) {}" iterates over the keys and values, and is generally not useful ($kv would be a key, then a value, and so on).

• To iterate over the indices of an array, use "foreach my $i (0 .. $#array) {}". "foreach my $v (@array) {}" iterates over the values.
• Perl requires braces following "if", "while", "foreach", etc.
• In Perl, "else if" is spelled "elsif".
• "? :" has higher precedence than assignment. In JavaScript, one can write:

      condition ? do_something() : variable = 3
      

  and the variable is only assigned if the condition is false. In Perl, you need parentheses:

      $condition ? do_something() : ($variable = 3);
      

  Or just use "if".

• Perl requires semicolons to separate statements.
• Variables declared with "my" only affect code after the declaration. You cannot write "$x = 1; my $x;" and expect the first assignment to affect the same variable. It will instead assign to an $x declared previously in an outer scope, or to a global variable.

  Note also that the variable is not visible until the following statement. This means that in "my $x = 1 + $x" the second $x refers to one declared previously.

• "my" variables are scoped to the current block, not to the current function. If you write "{my $x;} $x;", the second $x does not refer to the one declared inside the block.
• An object's members cannot be made accessible as variables. The closest Perl equivalent to "with(object) { method() }" is "for", which can alias $_ to the object:

      for ($object) {
          $_->method;
      }
      

• The object or class on which a method is called is passed as one of the method's arguments, not as a separate "this" value.

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 patterns.
• 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.