PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)NAMEperltrap - Perl traps for the unwary
DESCRIPTION
The biggest trap of all is forgetting to "use warnings" or
use the -w switch; see the perllexwarn manpage and the
perlrun manpage. 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 the perldelta manpage.
Awk Traps
Accustomed awk users should take special note of the fol
lowing:
The English module, loaded via
use English;
allows you to refer to special variables (like "$/")
with names (like $RS), as though they were in awk; see
the perlvar manpage for details.
Semicolons are required after all simple statements in
Perl (except at the end of a block). Newline is not a
statement delimiter.
Curly brackets are required on "if"s and "while"s.
Variables begin with "$", "@" 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 the perlvar
manpage.
$<digit> does not refer to fields--it refers to sub
strings matched by the last match pattern.
The print() statement does not add field and record
separators unless you set "$," 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 dif
ferently.
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 $RS to a pattern, only a string.
When in doubt, run the awk construct through a2p and
see what it gives you.
C Traps
Cerebral C programmers should take note of the following:
Curly brackets are required on "if"'s and "while"'s.
You must use "elsif" rather than "else if".
The "break" and "continue" keywords from C become in
Perl "last" and "next", respectively. Unlike in C,
these do not work within a "do { } while" construct.
There's no switch statement. (But it's easy to build
one on the fly.)
Variables begin with "$", "@" or "%" in Perl.
Comments begin with "#", not "/*".
You can't take the address of anything, although a
similar operator in Perl is the backslash, which cre
ates a reference.
"ARGV" must be capitalized. "$ARGV[0]" is C's
"argv[1]", and "argv[0]" ends up in "$0".
System calls such as link(), unlink(), rename(), etc.
return nonzero for success, not 0. (system(), however,
returns zero for success.)
Signal handlers deal with signal names, not numbers.
Use "kill -l" to find their names on your system.
Sed Traps
Seasoned sed programmers should take note of the follow
ing:
Backreferences in substitutions use "$" 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 substitu
tion on each command line. Perl does substitution in
only certain constructs such as double quotes, back
ticks, 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.
Perl Traps
Practicing Perl Programmers should take note of the fol
lowing:
Remember that many operations behave differently in a
list context than they do in a scalar one. See the
perldata manpage 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 the perlop
manpage and the perlsub manpage.
People have a hard time remembering that some func
tions 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 the perlform manpage 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.
Perl4 to Perl5 Traps
Practicing Perl4 Programmers should take note of the fol
lowing Perl4-to-Perl5 specific traps.
They're crudely ordered according to the following list:
Discontinuance, Deprecation, and BugFix traps
Anything that's been fixed as a perl4 bug, removed as
a perl4 feature or deprecated as a perl4 feature with
the intent to encourage usage of some other perl5 fea
ture.
Parsing Traps
Traps that appear to stem from the new parser.
Numerical Traps
Traps having to do with numerical or mathematical
operators.
General data type traps
Traps involving perl standard data types.
Context Traps - scalar, list contexts
Traps related to context within lists, scalar state
ments/declarations.
Precedence Traps
Traps related to the precedence of parsing, evalua
tion, and execution of code.
General Regular Expression Traps using s///, etc.
Traps related to the use of pattern matching.
Subroutine, Signal, Sorting Traps
Traps related to the use of signals and signal han
dlers, general subroutines, and sorting, along with
sorting subroutines.
OS Traps
OS-specific traps.
DBM Traps
Traps specific to the use of "dbmopen()", and specific
dbm implementations.
Unclassified Traps
Everything else.
If you find an example of a conversion trap that is not
listed here, please submit it to <perlbug@perl.org> for
inclusion. Also note that at least some of these can be
caught with the "use warnings" pragma or the -w switch.
Discontinuance, Deprecation, and BugFix traps
Anything that has been discontinued, deprecated, or fixed
as a bug from perl4.
Discontinuance
Symbols starting with "_" are no longer forced into
package main, except for "$_" itself (and "@_", etc.).
package test;
$_legacy = 1;
package main;
print "\$_legacy is ",$_legacy,"\n";
# perl4 prints: $_legacy is 1
# perl5 prints: $_legacy is
Deprecation
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
Given that "::" is now the preferred package delim
iter, it is debatable whether this should be classed
as a bug or not. (The older package delimiter, ' ,is
used here)
$x = 10 ;
print "x=${'x}\n" ;
# perl4 prints: x=10
# perl5 prints: Can't find string terminator "'" anywhere before EOF
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 "$:".
BugFix
The second and third arguments of "splice()" are now
evaluated in scalar context (as the Camel says) rather
than list context.
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
Discontinuance
You can't do a "goto" into a block that is optimized
away. Darn.
goto marker1;
for(1){
marker1:
print "Here I is!\n";
}
# perl4 prints: Here I is!
# perl5 errors: Can't "goto" into the middle of a foreach loop
Discontinuance
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.
$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: Bareword found where operator expected
Discontinuance
The archaic while/if BLOCK BLOCK syntax is no longer
supported.
if { 1 } {
print "True!";
}
else {
print "False!";
}
# perl4 prints: True!
# perl5 errors: syntax error at test.pl line 1, near "if {"
BugFix
The "**" 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
Discontinuance
The meaning of "foreach{}" has changed slightly when
it is iterating over a list which is not an array.
This used to assign the list to a temporary array, but
no longer does so (for efficiency). This means that
you'll now be iterating over the actual values, not
over copies of the values. Modifications to the loop
variable can change the original values.
@list = ('ab','abc','bcd','def');
foreach $var (grep(/ab/,@list)){
$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 example, you might need to change
foreach $var (grep(/ab/,@list)){
to
foreach $var (@tmp = grep(/ab/,@list)){
Otherwise changing $var will clobber the values of
@list. (This most often happens when you use "$_" for
the loop variable, and call subroutines in the loop
that don't properly localize "$_".)
Discontinuance
"split" with no arguments now behaves like "split ' '"
(which doesn't return an initial null field if $_
starts with whitespace), it used to behave like "split
/\s+/" (which does).
$_ = ' hi mom';
print join(':', split);
# perl4 prints: :hi:mom
# perl5 prints: hi:mom
BugFix
Perl 4 would ignore any text which was attached to an
-e switch, always taking the code snippet from the
following arg. Additionally, it would silently accept
an -e switch without a following arg. Both of 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.
Discontinuance
In Perl 4 the return value of "push" was undocumented,
but it was actually the last value being pushed onto
the target list. In Perl 5 the return value of "push"
is documented, but has changed, it is the number of
elements in the resulting list.
@x = ('existing');
print push(@x, 'first new', 'second new');
# perl4 prints: second new
# perl5 prints: 3
Deprecation
Some error messages will be different.
Discontinuance
In Perl 4, if in list context the delimiters to the
first argument of "split()" were "??", the result
would be placed in "@_" as well as being returned.
Perl 5 has more respect for your subroutine arguments.
Discontinuance
Some bugs may have been inadvertently removed. :-)
Parsing Traps
Perl4-to-Perl5 traps from having to do with parsing.
Parsing
Note the space between . and =
$string . = "more string";
print $string;
# perl4 prints: more string
# perl5 prints: syntax error at - line 1, near ". ="
Parsing
Better parsing in perl 5
sub foo {}
&foo
print("hello, world\n");
# perl4 prints: hello, world
# perl5 prints: syntax error
Parsing
"if it looks like a function, it is a function" rule.
print
($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
Parsing
String interpolation of the "$#array" construct dif
fers when braces are to used around the name.
@a = (1..3);
print "${#a}";
# perl4 prints: 2
# perl5 fails with syntax error
@ = (1..3);
print "$#{a}";
# perl4 prints: {a}
# perl5 prints: 2
Numerical Traps
Perl4-to-Perl5 traps having to do with numerical opera
tors, operands, or output from same.
Numerical
Formatted output and significant digits
print 7.373504 - 0, "\n";
printf "%20.18f\n", 7.373504 - 0;
# Perl4 prints:
7.375039999999996141
7.37503999999999614
# Perl5 prints:
7.373504
7.37503999999999614
Numerical
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 in
version 5.003_04. But always be wary when using
large integers. If in doubt:
use Math::BigInt;
Numerical
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
$p = ($test == 1);
print $p,"\n";
# perl4 prints: 0
# perl5 prints:
Also see the section on "General Regular Expression
Traps using s///, etc." for another example of this
new feature...
Bitwise string ops
When bitwise operators which can operate upon either
numbers or strings ("& | ^ ~") are given only strings
as arguments, perl4 would treat the operands as bit
strings so long as the program contained a call to
the "vec()" function. perl5 treats the string
operands as bitstrings. (See the Bitwise String
Operators entry in the perlop manpage 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
General data type traps
Perl4-to-Perl5 traps involving most data-types, and their
usage within certain expressions and/or context.
(Arrays)
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 $a[-2] \n";
# perl4 prints: The third element of the array is 4 also expressed as
# perl5 prints: The third element of the array is 4 also expressed as 4
(Arrays)
Setting "$#array" lower now discards array elements,
and makes them impossible to recover.
@a = (a,b,c,d,e);
print "Before: ",join('',@a);
$#a =1;
print ", After: ",join('',@a);
$#a =3;
print ", Recovered: ",join('',@a),"\n";
# perl4 prints: Before: abcde, After: ab, Recovered: abcd
# perl5 prints: Before: abcde, After: ab, Recovered: ab
(Hashes)
Hashes get defined before use
local($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
Perl will now generate a warning when it sees
defined(@a) and defined(%h).
(Globs)
glob assignment from variable to variable will fail
if the assigned variable is localized subsequent to
the assignment
@a = ("This is Perl 4");
*b = *a;
local(@a);
print @b,"\n";
# perl4 prints: This is Perl 4
# perl5 prints:
(Globs)
Assigning "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 "undef" is assigned to a typeglob. (Note
that assigning "undef" to a typeglob is different
than calling the "undef" function on a typeglob
("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
(Scalar String)
Changes in unary negation (of strings) This change
effects both the return value and what it does to
auto(magic)increment.
$x = "aaa";
print ++$x," : ";
print -$x," : ";
print ++$x,"\n";
# perl4 prints: aab : -0 : 1
# perl5 prints: aab : -aab : aac
(Constants)
perl 4 lets you modify constants:
$foo = "x";
&mod($foo);
for ($x = 0; $x < 3; $x++) {
&mod("a");
}
sub mod {
print "before: $_[0]";
$_[0] = "m";
print " after: $_[0]\n";
}
# perl4:
# before: x after: m
# before: a after: m
# before: m after: m
# before: m after: m
# Perl5:
# before: x after: m
# Modification of a read-only value attempted at foo.pl line 12.
# before: a
(Scalars)
The behavior is slightly different for:
print "$x", defined $x
# perl 4: 1
# perl 5: <no output, $x is not called into existence>
(Variable Suicide)
Variable suicide behavior is more consistent under
Perl 5. Perl5 exhibits the same behavior for hashes
and scalars, that perl4 exhibits for only scalars.
$aGlobal{ "aKey" } = "global value";
print "MAIN:", $aGlobal{"aKey"}, "\n";
$GlobalLevel = 0;
&test( *aGlobal );
sub test {
local( *theArgument ) = @_;
local( %aNewLocal ); # perl 4 != 5.001l,m
$aNewLocal{"aKey"} = "this should never appear";
print "SUB: ", $theArgument{"aKey"}, "\n";
$aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
$GlobalLevel++;
if( $GlobalLevel<4 ) {
&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
# SUB: this should never appear
# SUB: this should never appear
# SUB: this should never appear
Context Traps - scalar, list contexts
(list context)
The elements of argument lists for formats are now
evaluated in list context. This means you can inter
polate list values now.
@fmt = ("foo","bar","baz");
format STDOUT=
@<<<<< @||||| @>>>>>
@fmt;
.
write;
# perl4 errors: Please use commas to separate fields in file
# perl5 prints: foo bar baz
(scalar context)
The "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
(scalar context)
The comma operator in a scalar context is now guaran
teed to give a scalar context to its arguments.
@y= ('a','b','c');
$x = (1, 2, @y);
print "x = $x\n";
# Perl4 prints: x = c # Thinks list context interpolates list
# Perl5 prints: x = 3 # Knows scalar uses length of list
(list, builtin)
"sprintf()" is prototyped as ($;@), 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);
print $x;
# perl4 prints: foobar
# perl5 prints: 3
"printf()" works the same as it did in Perl 4,
though:
@z = ('%s%s', 'foo', 'bar');
printf STDOUT (@z);
# perl4 prints: foobar
# perl5 prints: foobar
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.
Precedence
LHS vs. RHS of any assignment operator. LHS is eval
uated 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;
print join( ' ', keys %a );
# perl4 prints: left
# perl5 prints: right
Precedence
These are now semantic errors because of precedence:
@list = (1,2,3,4,5);
%map = ("a",1,"b",2,"c",3,"d",4);
$n = shift @list + 2; # first item in list plus 2
print "n is $n, ";
$m = keys %map + 2; # number of items in hash plus 2
print "m is $m\n";
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
Precedence
The precedence of assignment operators is now the
same as the precedence of assignment. Perl 4 mistak
enly gave them the precedence of the associated oper
ator. So you now must parenthesize them in expres
sions like
/foo/ ? ($a += 2) : ($a -= 2);
Otherwise
/foo/ ? $a += 2 : $a -= 2
would be erroneously parsed as
(/foo/ ? $a += 2 : $a) -= 2;
On the other hand,
$a += /foo/ ? 1 : 2;
now works as a C programmer would expect.
Precedence
open FOO || die;
is now incorrect. You need parentheses around the
filehandle. Otherwise, perl5 leaves the statement as
its default precedence:
open(FOO || die);
# perl4 opens or dies
# perl5 opens FOO, dying only if 'FOO' is false, i.e. never
Precedence
perl4 gives the special variable, "$:" precedence,
where perl5 treats "$::" as main "package"
$a = "x"; print "$::a";
# perl 4 prints: -:a
# perl 5 prints: x
Precedence
perl4 had buggy precedence for the file test opera
tors vis-a-vis the assignment operators. Thus,
although the precedence table for perl4 leads one to
believe "-e $foo .= "q"" should parse as "((-e $foo)
.= "q")", it actually parses as "(-e ($foo .= "q"))".
In perl5, the precedence is as documented.
-e $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
Precedence
In perl4, keys(), each() and values() were special
high-precedence operators that operated on a single
hash, but in perl5, they are regular named unary
operators. As documented, named unary operators have
lower precedence than the arithmetic and concatena
tion operators "+ - .", but the perl4 variants of
these operators actually bind tighter than "+ - .".
Thus, for:
%foo = 1..10;
print keys %foo - 1
# perl4 prints: 4
# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
The perl4 behavior was probably more useful, if less
consistent.
General Regular Expression Traps using s///, etc.
All types of RE traps.
Regular Expression
"s'$lhs'$rhs'" now does no interpolation on either
side. It used to 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
Regular Expression
"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 state of the
searched string is lost)
$_ = "ababab";
while(m/ab/g){
&doit("blah");
}
sub doit{local($_) = shift; print "Got $_ "}
# perl4 prints: Got blah Got blah Got blah Got blah
# perl5 prints: infinite loop blah...
Regular Expression
Currently, if you use the "m//o" qualifier on a regu
lar expression within an anonymous sub, all closures
generated from that anonymous sub will use the regu
lar expression as it was compiled when it was used
the very first time in any such closure. For
instance, if you say
sub build_match {
my($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 $right as they were the
first time that build_match() was called, not as they
are in the current call.
Regular Expression
If no parentheses are used in a match, Perl4 sets
"$+" to the whole match, just like "$&". Perl5 does
not.
"abcdef" =~ /b.*e/;
print "\$+ = $+\n";
# perl4 prints: bcde
# perl5 prints:
Regular Expression
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 the Numerical Traps entry elsewhere in this
document for another example of this new feature.
Regular Expression
"s`lhs`rhs`" (using backticks) is now a normal sub
stitution, with no backtick expansion
$string = "";
$string =~ s`^`hostname`;
print $string, "\n";
# perl4 prints: <the local hostname>
# perl5 prints: hostname
Regular Expression
Stricter parsing of variables used in regular expres
sions
s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
# perl4: compiles w/o error
# perl5: with Scalar found where operator expected ..., near "$opt$plus"
an added component of this example, apparently from
the same script, is the actual value of the s'd
string after the substitution. "[$opt]" is a charac
ter class in perl4 and an array subscript in perl5
$grpc = 'a';
$opt = 'r';
$_ = 'bar';
s/^([^$grpc]*$grpc[$opt]?)/foo/;
print ;
# perl4 prints: foo
# perl5 prints: foobar
Regular Expression
Under perl5, "m?x?" matches only once, like "?x?".
Under perl4, it matched repeatedly, like "/x/" or
"m!x!".
$test = "once";
sub match { $test =~ m?once?; }
&match();
if( &match() ) {
# m?x? matches more then once
print "perl4\n";
} else {
# m?x? matches only once
print "perl5\n";
}
# perl4 prints: perl4
# perl5 prints: perl5
Subroutine, Signal, Sorting Traps
The general group of Perl4-to-Perl5 traps having to do
with Signals, Sorting, and their related subroutines, as
well as general subroutine traps. Includes some OS-Spe
cific traps.
(Signals)
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 now main'SeeYa
# perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
Use -w to catch this one
(Sort Subroutine)
reverse is no longer allowed as the name of a sort
subroutine.
sub reverse{ print "yup "; $a <=> $b }
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()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
OS Traps
(SysV)
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 @_... ";
}
$SIG{'INT'} = 'gotit';
$| = 1;
$pid = fork;
if ($pid) {
kill('INT', $pid);
sleep(1);
kill('INT', $pid);
} else {
while (1) {sleep(10);}
}
# perl4 (HPUX) prints: Got INT...
# perl5 (HPUX) prints: Got INT... Got INT...
(SysV)
Under SysV OSes, "seek()" on a file opened to append
">>" now does the right thing w.r.t. the fopen() man
page. e.g., - When a file is opened for append, it
is impossible to overwrite information already in
the file.
open(TEST,">>seek.test");
$start = tell TEST ;
foreach(1 .. 9){
print TEST "$_ ";
}
$end = tell TEST ;
seek(TEST,$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
Interpolation Traps
Perl4-to-Perl5 traps having to do with how things get
interpolated within certain expressions, statements, con
texts, or whatever.
Interpolation
@ now always interpolates an array in double-quotish
strings.
print "To: someone@somewhere.com\n";
# perl4 prints: To:someone@somewhere.com
# 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
Interpolation
Double-quoted strings may no longer end with an
unescaped $ or @.
$foo = "foo$";
$bar = "bar@";
print "foo is $foo, bar is $bar\n";
# perl4 prints: foo is foo$, bar is bar@
# perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in
$bar
Interpolation
Perl now sometimes evaluates arbitrary expressions
inside braces that occur within double quotes (usu
ally when the opening brace is preceded by "$" or
"@").
@www = "buz";
$foo = "foo";
$bar = "bar";
sub foo { return "bar" };
print "|@{w.w.w}|${main'foo}|";
# perl4 prints: |@{w.w.w}|foo|
# perl5 prints: |buz|bar|
Note that you can "use strict;" to ward off such
trappiness under perl5.
Interpolation
The construct "this is $$x" used to interpolate the
pid at that point, but now tries to dereference $x.
"$$" 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 a reference
Interpolation
Creation of hashes on the fly with "eval "EXPR"" now
requires either both "$"'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 "eval{}" if possible.
$hashname = "foobar";
$key = "baz";
$value = 1234;
eval "\$$hashname{'$key'} = q|$value|";
(defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
# perl4 prints: Yup
# perl5 prints: Nope
Changing
eval "\$$hashname{'$key'} = q|$value|";
to
eval "\$\$hashname{'$key'} = q|$value|";
causes the following result:
# perl4 prints: Nope
# perl5 prints: Yup
or, changing to
eval "\$$hashname\{'$key'\} = q|$value|";
causes the following result:
# perl4 prints: Yup
# perl5 prints: Yup
# and is compatible for both versions
Interpolation
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
Interpolation
You also have to be careful about array references.
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
Interpolation
Similarly, watch out for:
$foo = "baz";
print "\$$foo{bar}\n";
# perl4 prints: $baz{bar}
# perl5 prints: $
Perl 5 is looking for "$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
"eval"'s.
Interpolation
"qq()" string passed to "eval"
eval qq(
foreach \$y (keys %\$x\) {
\$count++;
}
);
# perl4 runs this ok
# perl5 prints: Can't find string terminator ")"
DBM Traps
General DBM traps.
DBM
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 must have
been linked with the same dbm/ndbm as the default for
"dbmopen()" to function properly without "tie"'ing to
an extension dbm implementation.
dbmopen (%dbm, "file", undef);
print "ok\n";
# perl4 prints: ok
# perl5 prints: ok (IFF linked with -ldbm or -lndbm)
DBM
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 when
exceeding the limit on the key/value size will cause
perl5 to exit immediately.
dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
$DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
print "YUP\n";
# perl4 prints:
dbm store returned -1, errno 28, key "trap" at - line 3.
YUP
# perl5 prints:
dbm store returned -1, errno 28, key "trap" at - line 3.
Unclassified Traps
Everything else.
""require""/""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:
return 3;
Running doit.pl gives the following:
# perl 4 prints: 3 (aborts the subroutine early)
# perl 5 prints: 8
Same behavior if you replace "do" with "require".
""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.
As always, if any of these are ever officially declared as
bugs, they'll be fixed and removed.
2001-03-18 perl v5.6.1 PERLTRAP(1)
