TRAPS

Accustomed awk users should take special note of the following:

• 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 ifs and whiles.
• Variables begin with $ or @ in perl.
• Arrays index from 0 unless you set $[. Likewise string positions in substr() and index().
• You have to decide whether your array has numeric or string indices.
• Associative array 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 yourself to an array. And the split operator has different arguments.
• 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.)
• $ 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 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.)
• The concatenation operator is ".", not the null string. (Using the null string would render "/pat/ /pat/" unparsable, since the third slash would be interpreted as a division operator--the tokener is in fact slightly context sensitive for operators like /, ?, and <. And in fact, . itself can be the beginning of a number.)
• Next, exit and continue work differently.
• The following variables work differently

            Awk                   Perl
            ARGC                  $#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                $;
  

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

Cerebral C programmers should take note of the following:

• Curly brackets are required on ifs and whiles.
• You should use "elsif" rather than "else if"
• Break and continue become last and next, respectively.
• There's no switch statement.
• Variables begin with $ or @ in perl.
• Printf does not implement *.
• Comments begin with #, not /*.
• You can't take the address of anything.
• ARGV must be capitalized.
• The "system" calls link, unlink, rename, etc. return nonzero for success, not 0.
• Signal handlers deal with signal names, not numbers.

Seasoned sed programmers should take note of the following:

• Backreferences in substitutions use $ rather than \.
• The pattern matching metacharacters (, ), and | do not have backslashes in front.
• The range operator is .. rather than comma.

Sharp shell programmers should take note of the following:

• The backtick operator does variable interpretation 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 only in certain constructs such as double quotes, backticks, angle brackets and search patterns.
• Shells interpret scripts a little bit at a time. Perl compiles the whole program before executing it.
• The arguments are available via @ARGV, not $1, $2, etc.
• The environment is not automatically made available as variables.