=head1 DESCRIPTION
-The biggest trap of all is forgetting to use the B<-w> switch; see
-L<perlrun>. The second biggest trap is not making your entire program
-runnable under C<use strict>.
+The biggest trap of all is forgetting to C<use warnings> or use the B<-w>
+switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not
+making your entire program runnable under C<use strict>. The third biggest
+trap is not reading the list of changes in this version of Perl; see
+L<perldelta>.
=head2 Awk Traps
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
The English module, loaded via
use English;
-allows you to refer to special variables (like $RS) as
-though they were in B<awk>; see L<perlvar> for details.
+allows you to refer to special variables (like C<$/>) with names (like
+
$RS), as though they were in B<awk>; see L<perlvar> for details.
=item *
=item *
-Variables begin with "$" or "@" in Perl.
+Variables begin with "$", "@" or "%" in Perl.
=item *
=item *
Reading an input line does not split it for you. You get to split it
-yourself to an array. And the split() operator has different
-arguments.
+to an array yourself. And the split() operator has different
+arguments than B<awk>'s.
=item *
=item *
-$E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched
+$<I<digit>> does not refer to fields--it refers to substrings matched
by the last match pattern.
=item *
The concatenation operator is ".", not the null string. (Using the
null string would render C</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 "E<gt>".
+slightly context sensitive for operators like "/", "?", and ">".
And in fact, "." itself can be the beginning of a number.)
=item *
The following variables work differently:
Awk Perl
- ARGC $#ARGV or scalar @ARGV
+ ARGC scalar @ARGV (compare with $#ARGV)
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
=back
-=head2 C Traps
+=head2 C/C++ Traps
-Cerebral C programmers should take note of the following:
+Cerebral C and C++ programmers should take note of the following:
=over 4
=item *
-The C<break> and C<continue> keywords from C become in
-Perl C<last> and C<next>, respectively.
-Unlike in C, these do I<NOT> work within a C<do { } while> construct.
+The C<break> and C<continue> keywords from C become in Perl C<last>
+and C<next>, respectively. Unlike in C, these do I<not> work within a
+C<do { } while> construct. See L<perlsyn/"Loop Control">.
=item *
-There's no switch statement. (But it's easy to build one on the fly.)
+The switch statement is called C<given/when> and only available in
+perl 5.10 or newer. See L<perlsyn/"Switch statements">.
=item *
-Variables begin with "$" or "@" in Perl.
+Variables begin with "$", "@" or "%" in Perl.
=item *
-C<printf()> does not implement the "*" format for interpolating
-field widths, but it's trivial to use interpolation of double-quoted
-strings to achieve the same effect.
-
-=item *
-
-Comments begin with "#", not "/*".
+Comments begin with "#", not "/*" or "//". Perl may interpret C/C++
+comments as division operators, unterminated regular expressions or
+the defined-or operator.
=item *
=item *
System calls such as link(), unlink(), rename(), etc. return nonzero for
-success, not 0.
+success, not 0. (system(), however, returns zero for success.)
=item *
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
Backreferences in substitutions use "$" rather than "\".
=item *
=item *
-The back-tick operator does variable interpolation without regard to
+The backtick operator does variable interpolation without regard to
the presence of single quotes in the command.
=item *
-The back-tick operator does no translation of the return value, unlike B<csh>.
+The backtick operator does no translation of the return value, unlike B<csh>.
=item *
Shells (especially B<csh>) do several levels of substitution on each
command line. Perl does substitution in only certain constructs
-such as double quotes, back-ticks, angle brackets, and search patterns.
+such as double quotes, backticks, angle brackets, and search patterns.
=item *
The environment is not automatically made available as separate scalar
variables.
+=item *
+
+The shell's C<test> uses "=", "!=", "<" etc for string comparisons and "-eq",
+"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
+uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc
+for numeric comparisons.
+
=back
=head2 Perl Traps
=item *
-Avoid barewords if you can, especially all lower-case 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
+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.
=item *
-You cannot discern from mere inspection which built-ins
-are unary operators (like chop() and chdir())
+You cannot discern from mere inspection which builtins
+are unary operators (like chop() and chdir())
and which are list operators (like print() and unlink()).
-(User-defined subroutines can be B<only> list operators, never
-unary ones.) See L<perlop>.
+(Unless prototyped, user-defined subroutines can B<only> be list
+operators, never unary ones.) See L<perlop> and L<perlsub>.
=item *
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.
+you might expect to do not.
=item *
-The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
+The <FH> construct is not the name of the filehandle, it is a readline
operation on that handle. The data read is assigned to $_ only if the
file read is the sole condition in a while loop:
while (<FH>) { }
- while ($_ = <FH>) { }..
+ while (defined($_ = <FH>)) { }..
<FH>; # data discarded!
=item *
-Remember not to use "C<=>" when you need "C<=~>";
+Remember not to use C<=> when you need C<=~>;
these two constructs are quite different:
$x = /foo/;
=item *
-The C<do {}> construct isn't a real loop that you can use
+The C<do {}> construct isn't a real loop that you can use
loop control on.
=item *
-Use C<my()> for local variables whenever you can get away with
-it (but see L<perlform> for where you can't).
-Using C<local()> actually gives a local value to a global
+Use C<my()> for local variables whenever you can get away with
+it (but see L<perlform> for where you can't).
+Using C<local()> actually gives a local value to a global
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.
=head2 Perl4 to Perl5 Traps
-Practicing Perl4 Programmers should take note of the following
+Practicing Perl4 Programmers should take note of the following
Perl4-to-Perl5 specific traps.
They're crudely ordered according to the following list:
=back
If you find an example of a conversion trap that is not listed here,
-please submit it to Bill Middleton <F<wjm@best.com>> for inclusion.
-Also note that at least some of these can be caught with B<-w>.
+please submit it to <F<perlbug@perl.org>> for inclusion.
+Also note that at least some of these can be caught with the
+C<use warnings> pragma or the B<-w> switch.
=head2 Discontinuance, Deprecation, and BugFix traps
Anything that has been discontinued, deprecated, or fixed as
-a bug from perl4.
+a bug from perl4.
=over 4
-=item * Discontinuance
+=item * Symbols starting with "_" no longer forced into main
Symbols starting with "_" are no longer forced into package main, except
for C<$_> itself (and C<@_>, etc.).
package main;
print "\$_legacy is ",$_legacy,"\n";
-
+
# perl4 prints: $_legacy is 1
# perl5 prints: $_legacy is
-=item * Deprecation
+=item * Double-colon valid package separator in variable name
Double-colon is now a valid package separator in a variable name. Thus these
behave differently in perl4 vs. perl5, because the packages don't exist.
$a=1;$b=2;$c=3;$var=4;
print "$a::$b::$c ";
print "$var::abc::xyz\n";
-
+
# perl4 prints: 1::2::3 4::abc::xyz
# perl5 prints: 3
whether this should be classed as a bug or not.
(The older package delimiter, ' ,is used here)
- $x = 10 ;
- print "x=${'x}\n" ;
-
+ $x = 10;
+ print "x=${'x}\n";
+
# perl4 prints: x=10
# perl5 prints: Can't find string terminator "'" anywhere before EOF
-Also see precedence traps, for parsing C<$:>.
+You can avoid this problem, and remain compatible with perl4, if you
+always explicitly include the package name:
+
+ $x = 10;
+ print "x=${main'x}\n";
+
+Also see precedence traps, for parsing C<$:>.
-=item * BugFix
+=item * 2nd and 3rd args to C<splice()> are now in scalar context
The second and third arguments of C<splice()> are now evaluated in scalar
context (as the Camel says) rather than list context.
- sub sub1{return(0,2) } # return a 2-elem array
- sub sub2{ return(1,2,3)} # return a 3-elem array
- @a1 = ("a","b","c","d","e");
+ sub sub1{return(0,2) } # return a 2-element list
+ sub sub2{ return(1,2,3)} # return a 3-element list
+ @a1 = ("a","b","c","d","e");
@a2 = splice(@a1,&sub1,&sub2);
print join(' ',@a2),"\n";
-
+
# perl4 prints: a b
- # perl5 prints: c d e
+ # perl5 prints: c d e
-=item * Discontinuance
+=item * Can't do C<goto> into a block that is optimized away
You can't do a C<goto> into a block that is optimized away. Darn.
goto marker1;
- for(1){
+ for(1){
marker1:
print "Here I is!\n";
- }
-
+ }
+
# perl4 prints: Here I is!
- # perl5 dumps core (SEGV)
+ # perl5 errors: Can't "goto" into the middle of a foreach loop
-=item * Discontinuance
+=item * Can't use whitespace as variable name or quote delimiter
It is no longer syntactically legal to use whitespace as the name
of a variable, or as a delimiter for any kind of quote construct.
-Double darn.
+Double darn.
$a = ("foo bar");
$b = q baz ;
print "a is $a, b is $b\n";
-
+
# perl4 prints: a is foo bar, b is baz
- # perl5 errors: Bare word found where operator expected
+ # perl5 errors: Bareword found where operator expected
-=item * Discontinuance
+=item * C<while/if BLOCK BLOCK> gone
The archaic while/if BLOCK BLOCK syntax is no longer supported.
else {
print "False!";
}
-
+
# perl4 prints: True!
# perl5 errors: syntax error at test.pl line 1, near "if {"
-=item * BugFix
+=item * C<**> binds tighter than unary minus
The C<**> operator now binds more tightly than unary minus.
It was documented to work this way before, but didn't.
print -4**2,"\n";
-
+
# perl4 prints: 16
# perl5 prints: -16
-=item * Discontinuance
+=item * C<foreach> changed when iterating over a list
The meaning of C<foreach{}> has changed slightly when it is iterating over a
list which is not an array. This used to assign the list to a
$var = 1;
}
print (join(':',@list));
-
+
# perl4 prints: ab:abc:bcd:def
# perl5 prints: 1:1:bcd:def
To retain Perl4 semantics you need to assign your list
-explicitly to a temporary array and then iterate over that. For
+explicitly to a temporary array and then iterate over that. For
example, you might need to change
foreach $var (grep(/ab/,@list)){
happens when you use C<$_> for the loop variable, and call subroutines in
the loop that don't properly localize C<$_>.)
-=item * Discontinuance
+=item * C<split> with no args behavior changed
C<split> with no arguments now behaves like C<split ' '> (which doesn't
return an initial null field if $_ starts with whitespace), it used to
# perl4 prints: :hi:mom
# perl5 prints: hi:mom
-=item * BugFix
+=item * B<-e> behavior fixed
Perl 4 would ignore any text which was attached to an B<-e> switch,
always taking the code snippet from the following arg. Additionally, it
these behaviors have been fixed.
perl -e'print "attached to -e"' 'print "separate arg"'
-
+
# perl4 prints: separate arg
# perl5 prints: attached to -e
-
+
perl -e
# perl4 prints:
# perl5 dies: No code specified for -e.
-=item * Discontinuance
+=item * C<push> returns number of elements in resulting list
In Perl 4 the return value of C<push> was undocumented, but it was
actually the last value being pushed onto the target list. In Perl 5
@x = ('existing');
print push(@x, 'first new', 'second new');
-
+
# perl4 prints: second new
# perl5 prints: 3
-=item * Deprecation
+=item * Some error messages differ
Some error messages will be different.
-=item * Discontinuance
+=item * C<split()> honors subroutine args
+
+In Perl 4, if in list context the delimiters to the first argument of
+C<split()> were C<??>, the result would be placed in C<@_> as well as
+being returned. Perl 5 has more respect for your subroutine arguments.
+
+=item * Bugs removed
Some bugs may have been inadvertently removed. :-)
=over 4
-=item * Parsing
+=item * Space between . and = triggers syntax error
Note the space between . and =
$string . = "more string";
print $string;
-
+
# perl4 prints: more string
# perl5 prints: syntax error at - line 1, near ". ="
-=item * Parsing
+=item * Better parsing in perl 5
Better parsing in perl 5
sub foo {}
&foo
print("hello, world\n");
-
+
# perl4 prints: hello, world
# perl5 prints: syntax error
-=item * Parsing
+=item * Function parsing
"if it looks like a function, it is a function" rule.
print
($foo == 1) ? "is one\n" : "is zero\n";
-
+
# perl4 prints: is zero
# perl5 warns: "Useless use of a constant in void context" if using -w
+=item * String interpolation of C<$#array> differs
+
+String interpolation of the C<$#array> construct differs when braces
+are to used around the name.
+
+ @a = (1..3);
+ print "${#a}";
+
+ # perl4 prints: 2
+ # perl5 fails with syntax error
+
+ @a = (1..3);
+ print "$#{a}";
+
+ # perl4 prints: {a}
+ # perl5 prints: 2
+
+=item * Perl guesses on C<map>, C<grep> followed by C<{> if it starts BLOCK or hash ref
+
+When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{>
+starts a BLOCK or a hash reference. If it guesses wrong, it will report
+a syntax error near the C<}> and the missing (or unexpected) comma.
+
+Use unary C<+> before C<{> on a hash reference, and unary C<+> applied
+to the first thing in a BLOCK (after C<{>), for perl to guess right all
+the time. (See L<perlfunc/map>.)
+
=back
=head2 Numerical Traps
=over 5
-=item * Numerical
+=item * Formatted output and significant digits
-Formatted output and significant digits
+Formatted output and significant digits. In general, Perl 5
+tries to be more precise. For example, on a Solaris Sparc:
print 7.373504 - 0, "\n";
- printf "%20.18f\n", 7.373504 - 0;
-
+ printf "%20.18f\n", 7.373504 - 0;
+
# Perl4 prints:
- 7.375039999999996141
- 7.37503999999999614
-
+ 7.3750399999999996141
+ 7.375039999999999614
+
# Perl5 prints:
7.373504
- 7.37503999999999614
+ 7.375039999999999614
+
+Notice how the first result looks better in Perl 5.
+
+Your results may vary, since your floating point formatting routines
+and even floating point format may be slightly different.
-=item * Numerical
+=item * Auto-increment operator over signed int limit deleted
This specific item has been deleted. It demonstrated how the auto-increment
operator would not catch when a number went over the signed int limit. Fixed
use Math::BigInt;
-=item * Numerical
+=item * Assignment of return values from numeric equality tests doesn't work
Assignment of return values from numeric equality tests
does not work in perl5 when the test evaluates to false (0).
-Logical tests now return an null, instead of 0
+Logical tests now return a null, instead of 0
$p = ($test == 1);
print $p,"\n";
# perl4 prints: 0
# perl5 prints:
-Also see the L<General Regular Expression Traps using s///, etc.>
-tests for another example of this new feature...
+Also see L<"General Regular Expression Traps using s///, etc.">
+for another example of this new feature...
+
+=item * Bitwise string ops
+
+When bitwise operators which can operate upon either numbers or
+strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
+treat the operands as bitstrings so long as the program contained a call
+to the C<vec()> function. perl5 treats the string operands as bitstrings.
+(See L<perlop/Bitwise String Operators> for more details.)
+
+ $fred = "10";
+ $barney = "12";
+ $betty = $fred & $barney;
+ print "$betty\n";
+ # Uncomment the next line to change perl4's behavior
+ # ($dummy) = vec("dummy", 0, 0);
+
+ # Perl4 prints:
+ 8
+
+ # Perl5 prints:
+ 10
+
+ # If vec() is used anywhere in the program, both print:
+ 10
=back
=over 5
-=item * (Arrays)
+=item * Negative array subscripts now count from the end of array
Negative array subscripts now count from the end of the array.
@a = (1, 2, 3, 4, 5);
print "The third element of the array is $a[3] also expressed as $a[-2] \n";
-
+
# perl4 prints: The third element of the array is 4 also expressed as
# perl5 prints: The third element of the array is 4 also expressed as 4
-=item * (Arrays)
+=item * Setting C<$#array> lower now discards array elements
Setting C<$#array> lower now discards array elements, and makes them
impossible to recover.
- @a = (a,b,c,d,e);
+ @a = (a,b,c,d,e);
print "Before: ",join('',@a);
- $#a =1;
+ $#a =1;
print ", After: ",join('',@a);
$#a =3;
print ", Recovered: ",join('',@a),"\n";
-
+
# perl4 prints: Before: abcde, After: ab, Recovered: abcd
# perl5 prints: Before: abcde, After: ab, Recovered: ab
-=item * (Hashes)
+=item * Hashes get defined before use
Hashes get defined before use
- local($s,@a,%h);
+ local($s,@a,%h);
die "scalar \$s defined" if defined($s);
die "array \@a defined" if defined(@a);
die "hash \%h defined" if defined(%h);
-
+
# perl4 prints:
# perl5 dies: hash %h defined
-=item * (Globs)
+Perl will now generate a warning when it sees defined(@a) and
+defined(%h).
+
+=item * Glob assignment from localized variable to variable
glob assignment from variable to variable will fail if the assigned
variable is localized subsequent to the assignment
*b = *a;
local(@a);
print @b,"\n";
-
+
# perl4 prints: This is Perl 4
# perl5 prints:
-
- # Another example
-
- *fred = *barney; # fred is aliased to barney
- @barney = (1, 2, 4);
- # @fred;
- print "@fred"; # should print "1, 2, 4"
-
- # perl4 prints: 1 2 4
- # perl5 prints: In string, @fred now must be written as \@fred
-
-=item * (Scalar String)
+
+=item * Assigning C<undef> to glob
+
+Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
+it undefines the associated scalar (but may have other side effects
+including SEGVs). Perl 5 will also warn if C<undef> is assigned to a
+typeglob. (Note that assigning C<undef> to a typeglob is different
+than calling the C<undef> function on a typeglob (C<undef *foo>), which
+has quite a few effects.
+
+ $foo = "bar";
+ *foo = undef;
+ print $foo;
+
+ # perl4 prints:
+ # perl4 warns: "Use of uninitialized variable" if using -w
+ # perl5 prints: bar
+ # perl5 warns: "Undefined value assigned to typeglob" if using -w
+
+=item * Changes in unary negation (of strings)
Changes in unary negation (of strings)
This change effects both the return value and what it
print ++$x," : ";
print -$x," : ";
print ++$x,"\n";
-
+
# perl4 prints: aab : -0 : 1
# perl5 prints: aab : -aab : aac
-=item * (Constants)
+=item * Modifying of constants prohibited
perl 4 lets you modify constants:
$_[0] = "m";
print " after: $_[0]\n";
}
-
+
# perl4:
# before: x after: m
# before: a after: m
# before: m after: m
# before: m after: m
-
+
# Perl5:
# before: x after: m
# Modification of a read-only value attempted at foo.pl line 12.
# before: a
-=item * (Scalars)
+=item * C<defined $var> behavior changed
The behavior is slightly different for:
print "$x", defined $x
-
+
# perl 4: 1
# perl 5: <no output, $x is not called into existence>
-=item * (Variable Suicide)
+=item * Variable Suicide
Variable suicide behavior is more consistent under Perl 5.
Perl5 exhibits the same behavior for hashes and scalars,
sub test {
local( *theArgument ) = @_;
local( %aNewLocal ); # perl 4 != 5.001l,m
- $aNewLocal{"aKey"} = "this should never appear";
+ $aNewLocal{"aKey"} = "this should never appear";
print "SUB: ", $theArgument{"aKey"}, "\n";
$aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
$GlobalLevel++;
&test( *aNewLocal );
}
}
-
+
# Perl4:
# MAIN:global value
# SUB: global value
# SUB: level 0
# SUB: level 1
# SUB: level 2
-
+
# Perl5:
# MAIN:global value
# SUB: global value
=over 5
-=item * (list context)
+=item * Elements of argument lists for formats evaluated in list context
The elements of argument lists for formats are now evaluated in list
context. This means you can interpolate list values now.
@<<<<< @||||| @>>>>>
@fmt;
.
- write;
-
+ write;
+
# perl4 errors: Please use commas to separate fields in file
# perl5 prints: foo bar baz
-=item * (scalar context)
+=item * C<caller()> returns false value in scalar context if no caller present
-The C<caller()> function now returns a false value in a scalar context
-if there is no caller. This lets library files determine if they're
+The C<caller()> function now returns a false value in a scalar context
+if there is no caller. This lets library files determine if they're
being required.
caller() ? (print "You rang?\n") : (print "Got a 0\n");
-
+
# perl4 errors: There is no caller
# perl5 prints: Got a 0
-=item * (scalar context)
+=item * Comma operator in scalar context gives scalar context to args
The comma operator in a scalar context is now guaranteed to give a
scalar context to its arguments.
@y= ('a','b','c');
$x = (1, 2, @y);
print "x = $x\n";
-
+
# Perl4 prints: x = c # Thinks list context interpolates list
# Perl5 prints: x = 3 # Knows scalar uses length of list
-=item * (list, builtin)
+=item * C<sprintf()> prototyped as C<($;@)>
-C<sprintf()> funkiness (array argument converted to scalar array count)
-This test could be added to t/op/sprintf.t
+C<sprintf()> is prototyped as ($;@), so its first argument is given scalar
+context. Thus, if passed an array, it will probably not do what you want,
+unlike Perl 4:
@z = ('%s%s', 'foo', 'bar');
$x = sprintf(@z);
- if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
-
- # perl4 prints: ok 2
- # perl5 prints: not ok 2
+ print $x;
-C<printf()> works fine, though:
+ # perl4 prints: foobar
+ # perl5 prints: 3
+
+C<printf()> works the same as it did in Perl 4, though:
+ @z = ('%s%s', 'foo', 'bar');
printf STDOUT (@z);
- print "\n";
-
+
# perl4 prints: foobar
# perl5 prints: foobar
-Probably a bug.
-
=back
=head2 Precedence Traps
Perl4-to-Perl5 traps involving precedence order.
+Perl 4 has almost the same precedence rules as Perl 5 for the operators
+that they both have. Perl 4 however, seems to have had some
+inconsistencies that made the behavior differ from what was documented.
+
=over 5
-=item * Precedence
+=item * LHS vs. RHS of any assignment operator
-LHS vs. RHS when both sides are getting an op.
+LHS vs. RHS of any assignment operator. LHS is evaluated first
+in perl4, second in perl5; this can affect the relationship
+between side-effects in sub-expressions.
@arr = ( 'left', 'right' );
$a{shift @arr} = shift @arr;
# perl4 prints: left
# perl5 prints: right
-=item * Precedence
+=item * Semantic errors introduced due to precedence
These are now semantic errors because of precedence:
print "n is $n, ";
$m = keys %map + 2; # number of items in hash plus 2
print "m is $m\n";
-
+
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
-=item * Precedence
+=item * Precedence of assignment operators same as the precedence of assignment
The precedence of assignment operators is now the same as the precedence
of assignment. Perl 4 mistakenly gave them the precedence of the associated
On the other hand,
- $a += /foo/ ? 1 : 2;
+ $a += /foo/ ? 1 : 2;
now works as a C programmer would expect.
-=item * Precedence
+=item * C<open> requires parentheses around filehandle
open FOO || die;
Otherwise, perl5 leaves the statement as its default precedence:
open(FOO || die);
-
+
# perl4 opens or dies
- # perl5 errors: Precedence problem: open FOO should be open(FOO)
+ # perl5 opens FOO, dying only if 'FOO' is false, i.e. never
-=item * Precedence
+=item * C<$:> precedence over C<$::> gone
perl4 gives the special variable, C<$:> precedence, where perl5
treats C<$::> as main C<package>
$a = "x"; print "$::a";
-
+
# perl 4 prints: -:a
# perl 5 prints: x
-=item * Precedence
+=item * Precedence of file test operators documented
+
+perl4 had buggy precedence for the file test operators vis-a-vis
+the assignment operators. Thus, although the precedence table
+for perl4 leads one to believe C<-e $foo .= "q"> should parse as
+C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
+In perl5, the precedence is as documented.
-concatenation precedence over filetest operator?
+ -e $foo .= "q"
- -e $foo .= "q"
-
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
-=item * Precedence
+=item * C<keys>, C<each>, C<values> are regular named unary operators
-Assignment to value takes precedence over assignment to key in
-perl5 when using the shift operator on both sides.
+In perl4, keys(), each() and values() were special high-precedence operators
+that operated on a single hash, but in perl5, they are regular named unary
+operators. As documented, named unary operators have lower precedence
+than the arithmetic and concatenation operators C<+ - .>, but the perl4
+variants of these operators actually bind tighter than C<+ - .>.
+Thus, for:
- @arr = ( 'left', 'right' );
- $a{shift @arr} = shift @arr;
- print join( ' ', keys %a );
+ %foo = 1..10;
+ print keys %foo - 1
- # perl4 prints: left
- # perl5 prints: right
+ # perl4 prints: 4
+ # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
+
+The perl4 behavior was probably more useful, if less consistent.
=back
=over 5
-=item * Regular Expression
+=item * C<s'$lhs'$rhs'> interpolates on either side
C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
-interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
+interpolate $lhs but not $rhs. (And still does not match a literal
'$' in string)
$a=1;$b=2;
$string = '1 2 $a $b';
$string =~ s'$a'$b';
print $string,"\n";
-
+
# perl4 prints: $b 2 $a $b
# perl5 prints: 1 2 $a $b
-=item * Regular Expression
+=item * C<m//g> attaches its state to the searched string
C<m//g> now attaches its state to the searched string rather than the
regular expression. (Once the scope of a block is left for the sub, the
&doit("blah");
}
sub doit{local($_) = shift; print "Got $_ "}
-
- # perl4 prints: blah blah blah
+
+ # perl4 prints: Got blah Got blah Got blah Got blah
# perl5 prints: infinite loop blah...
-=item * Regular Expression
+=item * C<m//o> used within an anonymous sub
+
+Currently, if you use the C<m//o> qualifier on a regular expression
+within an anonymous sub, I<all> closures generated from that anonymous
+sub will use the regular expression as it was compiled when it was used
+the very first time in any such closure. For instance, if you say
+
+ sub build_match {
+ my($left,$right) = @_;
+ return sub { $_[0] =~ /$left stuff $right/o; };
+ }
+ $good = build_match('foo','bar');
+ $bad = build_match('baz','blarch');
+ print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
+ print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
+ print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
+
+For most builds of Perl5, this will print:
+ok
+not ok
+not ok
+
+build_match() will always return a sub which matches the contents of
+$left and $right as they were the I<first> time that build_match()
+was called, not as they are in the current call.
+
+=item * C<$+> isn't set to whole match
If no parentheses are used in a match, Perl4 sets C<$+> to
the whole match, just like C<$&>. Perl5 does not.
"abcdef" =~ /b.*e/;
print "\$+ = $+\n";
-
+
# perl4 prints: bcde
# perl5 prints:
-=item * Regular Expression
+=item * Substitution now returns null string if it fails
substitution now returns the null string if it fails
$string = "test";
$value = ($string =~ s/foo//);
print $value, "\n";
-
+
# perl4 prints: 0
# perl5 prints:
Also see L<Numerical Traps> for another example of this new feature.
-=item * Regular Expression
+=item * C<s`lhs`rhs`> is now a normal substitution
-C<s`lhs`rhs`> (using back-ticks) is now a normal substitution, with no
-back-tick expansion
+C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
+backtick expansion
$string = "";
$string =~ s`^`hostname`;
print $string, "\n";
-
+
# perl4 prints: <the local hostname>
# perl5 prints: hostname
-=item * Regular Expression
+=item * Stricter parsing of variables in regular expressions
Stricter parsing of variables used in regular expressions
s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
-
+
# perl4: compiles w/o error
# perl5: with Scalar found where operator expected ..., near "$opt$plus"
the actual value of the s'd string after the substitution.
C<[$opt]> is a character class in perl4 and an array subscript in perl5
- $grpc = 'a';
+ $grpc = 'a';
$opt = 'r';
$_ = 'bar';
s/^([^$grpc]*$grpc[$opt]?)/foo/;
- print ;
-
+ print;
+
# perl4 prints: foo
# perl5 prints: foobar
-=item * Regular Expression
+=item * C<m?x?> matches only once
Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
repeatedly, like C</x/> or C<m!x!>.
if( &match() ) {
# m?x? matches more then once
print "perl4\n";
- } else {
+ } else {
# m?x? matches only once
- print "perl5\n";
+ print "perl5\n";
}
-
+
# perl4 prints: perl4
# perl5 prints: perl5
+=item * Failed matches don't reset the match variables
-=item * Regular Expression
-
-Under perl4 and upto version 5.003, a failed C<m//g> match used to
-reset the internal iterator, so that subsequent C<m//g> match attempts
-began from the beginning of the string. In perl version 5.004 and later,
-failed C<m//g> matches do not reset the iterator position (which can be
-found using the C<pos()> function--see L<perlfunc/pos>).
-
- $test = "foop";
- for (1..3) {
- print $1 while ($test =~ /(o)/g);
- # pos $test = 0; # to get old behavior
- }
-
- # perl4 prints: oooooo
- # perl5.004 prints: oo
-
-You may always reset the iterator yourself as shown in the commented line
-to get the old behavior.
+Unlike in Ruby, failed matches in Perl do not reset the match variables
+($1, $2, ..., C<$`>, ...).
=back
=over 5
-=item * (Signals)
+=item * Barewords that used to look like strings look like subroutine calls
Barewords that used to look like strings to Perl will now look like subroutine
calls if a subroutine by that name is defined before the compiler sees them.
sub SeeYa { warn"Hasta la vista, baby!" }
$SIG{'TERM'} = SeeYa;
print "SIGTERM is now $SIG{'TERM'}\n";
-
- # perl4 prints: SIGTERM is main'SeeYa
- # perl5 prints: SIGTERM is now main::1
+
+ # perl4 prints: SIGTERM is now main'SeeYa
+ # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
Use B<-w> to catch this one
-=item * (Sort Subroutine)
+=item * Reverse is no longer allowed as the name of a sort subroutine
reverse is no longer allowed as the name of a sort subroutine.
sub reverse{ print "yup "; $a <=> $b }
- print sort reverse a,b,c;
-
- # perl4 prints: yup yup yup yup abc
- # perl5 prints: abc
+ print sort reverse (2,1,3);
+
+ # perl4 prints: yup yup 123
+ # perl5 prints: 123
+ # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()
-=item * warn() won't let you specify a filehandle.
+=item * C<warn()> won't let you specify a filehandle.
Although it _always_ printed to STDERR, warn() would let you specify a
filehandle in perl4. With perl5 it does not.
warn STDERR "Foo!";
# perl4 prints: Foo!
- # perl5 prints: String found where operator expected
+ # perl5 prints: String found where operator expected
=back
=over 5
-=item * (SysV)
+=item * SysV resets signal handler correctly
-Under HPUX, and some other SysV OS's, one had to reset any signal handler,
-within the signal handler function, each time a signal was handled with
-perl4. With perl5, the reset is now done correctly. Any code relying
+Under HPUX, and some other SysV OSes, one had to reset any signal handler,
+within the signal handler function, each time a signal was handled with
+perl4. With perl5, the reset is now done correctly. Any code relying
on the handler _not_ being reset will have to be reworked.
Since version 5.002, Perl uses sigaction() under SysV.
sub gotit {
- print "Got @_... ";
- }
+ print "Got @_... ";
+ }
$SIG{'INT'} = 'gotit';
-
+
$| = 1;
$pid = fork;
if ($pid) {
kill('INT', $pid);
sleep(1);
kill('INT', $pid);
- } else {
+ } else {
while (1) {sleep(10);}
- }
-
+ }
+
# perl4 (HPUX) prints: Got INT...
# perl5 (HPUX) prints: Got INT... Got INT...
-=item * (SysV)
+=item * SysV C<seek()> appends correctly
-Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
-the right thing w.r.t. the fopen() man page. e.g., - When a file is opened
+Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
+the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
for append, it is impossible to overwrite information already in
the file.
open(TEST,">>seek.test");
- $start = tell TEST ;
+ $start = tell TEST;
foreach(1 .. 9){
print TEST "$_ ";
}
- $end = tell TEST ;
+ $end = tell TEST;
seek(TEST,$start,0);
print TEST "18 characters here";
-
+
# perl4 (solaris) seek.test has: 18 characters here
# perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
=over 5
-=item * Interpolation
+=item * C<@> always interpolates an array in double-quotish strings
@ now always interpolates an array in double-quotish strings.
- print "To: someone@somewhere.com\n";
-
+ print "To: someone@somewhere.com\n";
+
# perl4 prints: To:someone@somewhere.com
- # perl5 errors : In string, @somewhere now must be written as \@somewhere
+ # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
+ # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
-=item * Interpolation
+=item * Double-quoted strings may no longer end with an unescaped $
-Double-quoted strings may no longer end with an unescaped $ or @.
+Double-quoted strings may no longer end with an unescaped $.
$foo = "foo$";
- $bar = "bar@";
- print "foo is $foo, bar is $bar\n";
-
- # perl4 prints: foo is foo$, bar is bar@
+ print "foo is $foo\n";
+
+ # perl4 prints: foo is foo$
# perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in $bar
-=item * Interpolation
+=item * Arbitrary expressions are evaluated inside braces within double quotes
Perl now sometimes evaluates arbitrary expressions inside braces that occur
within double quotes (usually when the opening brace is preceded by C<$>
Note that you can C<use strict;> to ward off such trappiness under perl5.
-=item * Interpolation
+=item * C<$$x> now tries to dereference $x
-The construct "this is $$x" used to interpolate the pid at that
-point, but now apparently tries to dereference C<$x>. C<$$> by itself still
-works fine, however.
+The construct "this is $$x" used to interpolate the pid at that point, but
+now tries to dereference $x. C<$$> by itself still works fine, however.
+ $s = "a reference";
+ $x = *s;
print "this is $$x\n";
# perl4 prints: this is XXXx (XXX is the current pid)
- # perl5 prints: this is
+ # perl5 prints: this is a reference
-=item * Interpolation
+=item * Creation of hashes on the fly with C<eval "EXPR"> requires protection
-Creation of hashes on the fly with C<eval "EXPR"> now requires either both
-C<$>'s to be protected in the specification of the hash name, or both curlies
+Creation of hashes on the fly with C<eval "EXPR"> now requires either both
+C<$>'s to be protected in the specification of the hash name, or both curlies
to be protected. If both curlies are protected, the result will be compatible
with perl4 and perl5. This is a very common practice, and should be changed
to use the block form of C<eval{}> if possible.
# and is compatible for both versions
-=item * Interpolation
+=item * Bugs in earlier perl versions
perl4 programs which unconsciously rely on the bugs in earlier perl versions.
perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
-
+
# perl4 prints: This is not perl5
# perl5 prints: This is perl5
-=item * Interpolation
+=item * Array and hash brackets during interpolation
-You also have to be careful about array references.
+You also have to be careful about array and hash brackets during
+interpolation.
+
+ print "$foo["
+
+ perl 4 prints: [
+ perl 5 prints: syntax error
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
-=item * Interpolation
+Perl 5 is expecting to find an index or key name following the respective
+brackets, as well as an ending bracket of the appropriate type. In order
+to mimic the behavior of Perl 4, you must escape the bracket like so.
+
+ print "$foo\[";
+ print "$foo\{";
-Similarly, watch out for:
+=item * Interpolation of C<\$$foo{bar}>
- $foo = "array";
+Similarly, watch out for: C<\$$foo{bar}>
+
+ $foo = "baz";
print "\$$foo{bar}\n";
-
- # perl4 prints: $array{bar}
+
+ # perl4 prints: $baz{bar}
# perl5 prints: $
-Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
-happy just to expand $foo to "array" by itself. Watch out for this
+Perl 5 is looking for C<$foo{bar}> which doesn't exist, but perl 4 is
+happy just to expand $foo to "baz" by itself. Watch out for this
especially in C<eval>'s.
-=item * Interpolation
+=item * C<qq()> string passed to C<eval> will not find string terminator
C<qq()> string passed to C<eval>
\$count++;
}
);
-
+
# perl4 runs this ok
- # perl5 prints: Can't find string terminator ")"
+ # perl5 prints: Can't find string terminator ")"
=back
=over 5
-=item * DBM
+=item * Perl5 must have been linked with same dbm/ndbm as the default for C<dbmopen()>
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail. The build of perl5
# perl5 prints: ok (IFF linked with -ldbm or -lndbm)
-=item * DBM
+=item * DBM exceeding limit on the key/value size will cause perl5 to exit immediately
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail. The error generated
=over 5
-=item * Unclassified
-
-C<require>/C<do> trap using returned value
+=item * C<require>/C<do> trap using returned value
If the file doit.pl has:
sub foo {
$rc = do "./do.pl";
return 8;
- }
+ }
print &foo, "\n";
And the do.pl file has the following single line:
Running doit.pl gives the following:
# perl 4 prints: 3 (aborts the subroutine early)
- # perl 5 prints: 8
+ # perl 5 prints: 8
Same behavior if you replace C<do> with C<require>.
+=item * C<split> on empty string with LIMIT specified
+
+ $string = '';
+ @list = split(/foo/, $string, 2)
+
+Perl4 returns a one element list containing the empty string but Perl5
+returns an empty list.
+
=back
-As always, if any of these are ever officially declared as bugs,
+As always, if any of these are ever officially declared as bugs,
they'll be fixed and removed.
https://perl5.git.perl.org / perl5.git / blobdiff