PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)NAMEperltrap - Perl traps for the unwary
DESCRIPTION
The biggest trap of all is forgetting to use the -w
switch; see 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
following:
o The English module, loaded via
use English;
allows you to refer to special variables (like $/)
with names (like $RS), as though they were in awk; see
the perlvar manpage for details.
o Semicolons are required after all simple statements in
Perl (except at the end of a block). Newline is not a
statement delimiter.
o Curly brackets are required on ifs and whiles.
o Variables begin with "$", "@" or "%" in Perl.
o Arrays index from 0. Likewise string positions in
substr() and index().
o You have to decide whether your array has numeric or
string indices.
o Hash values do not spring into existence upon mere
reference.
o You have to decide whether you want to use string or
numeric comparisons.
o 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.
o 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.
16/Sep/1999 perl 5.005, patch 03 1
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
o $<digit> does not refer to fields--it refers to
substrings matched by the last match pattern.
o 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.
o You must open your files before you print to them.
o The range operator is "..", not comma. The comma
operator works as in C.
o The match operator is "=~", not "~". ("~" is the
one's complement operator, as in C.)
o 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.)
o 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.)
o The next, exit, and continue keywords work
differently.
o The following variables work differently:
Awk Perl
ARGC $#ARGV or scalar @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 $;
o You cannot set $RS to a pattern, only a string.
o When in doubt, run the awk construct through a2p and
see what it gives you.
16/Sep/1999 perl 5.005, patch 03 2
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
C Traps
Cerebral C programmers should take note of the following:
o Curly brackets are required on if's and while's.
o You must use elsif rather than else if.
o The break and continue keywords from C become in Perl
last and next, respectively. Unlike in C, these do
NOT work within a do { } while construct.
o There's no switch statement. (But it's easy to build
one on the fly.)
o Variables begin with "$", "@" or "%" in Perl.
o 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.
o Comments begin with "#", not "/*".
o You can't take the address of anything, although a
similar operator in Perl is the backslash, which
creates a reference.
o ARGV must be capitalized. $ARGV[0] is C's argv[1],
and argv[0] ends up in $0.
o System calls such as link(), unlink(), rename(), etc.
return nonzero for success, not 0.
o Signal handlers deal with signal names, not numbers.
Use kill -l to find their names on your system.
Sed Traps
Seasoned sed programmers should take note of the
following:
o Backreferences in substitutions use "$" rather than
"\".
o The pattern matching metacharacters "(", ")", and "|"
do not have backslashes in front.
o The range operator is ..., rather than comma.
Shell Traps
Sharp shell programmers should take note of the following:
16/Sep/1999 perl 5.005, patch 03 3
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
o The backtick operator does variable interpolation
without regard to the presence of single quotes in the
command.
o The backtick operator does no translation of the
return value, unlike csh.
o 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.
o 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).
o The arguments are available via @ARGV, not $1, $2,
etc.
o The environment is not automatically made available as
separate scalar variables.
Perl Traps
Practicing Perl Programmers should take note of the
following:
o Remember that many operations behave differently in a
list context than they do in a scalar one. See the
perldata manpage for details.
o 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.
o 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 only list operators,
never unary ones.) See the perlop manpage.
o 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.
o The <FH> construct is not the name of the filehandle,
it is a readline operation on that handle. The data
read is assigned to $_ only if the file read is the
sole condition in a while loop:
16/Sep/1999 perl 5.005, patch 03 4
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
while (<FH>) { }
while (defined($_ = <FH>)) { }..
<FH>; # data discarded!
o Remember not to use "=" when you need "=~"; these two
constructs are quite different:
$x = /foo/;
$x =~ /foo/;
o The do {} construct isn't a real loop that you can use
loop control on.
o 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.
o 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
following 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
feature.
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
statements/declarations.
16/Sep/1999 perl 5.005, patch 03 5
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
Precedence Traps
Traps related to the precedence of parsing,
evaluation, 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
handlers, 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 Bill Middleton
<wjm@best.com> for inclusion. Also note that at least
some of these can be caught with -w.
Discontinuance, Deprecation, and BugFix traps
Anything that has been discontinued, deprecated, or fixed
as a bug from perl4.
o 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
o 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.
16/Sep/1999 perl 5.005, patch 03 6
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
$a=1;$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 delimiter,
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 $:.
o 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
o 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";
}
16/Sep/1999 perl 5.005, patch 03 7
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# perl4 prints: Here I is!
# perl5 dumps core (SEGV)
o 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
o 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 {"
o 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
o 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.
16/Sep/1999 perl 5.005, patch 03 8
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
@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 $_.)
o 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
o 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.
16/Sep/1999 perl 5.005, patch 03 9
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
o 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
o Discontinuance
In Perl 4 (and versions of Perl 5 before 5.004), '\r'
characters in Perl code were silently allowed,
although they could cause (mysterious!) failures in
certain constructs, particularly here documents. Now,
'\r' characters cause an immediate fatal error.
(Note: In this example, the notation \015 represents
the incorrect line ending. Depending upon your text
viewer, it will look different.)
print "foo";\015
print "bar";
# perl4 prints: foobar
# perl5.003 prints: foobar
# perl5.004 dies: Illegal character \015 (carriage return)
See the perldiag manpage for full details.
o Deprecation
Some error messages will be different.
o Discontinuance
Some bugs may have been inadvertently removed. :-)
Parsing Traps
Perl4-to-Perl5 traps from having to do with parsing.
o Parsing
Note the space between . and =
$string . = "more string";
print $string;
# perl4 prints: more string
# perl5 prints: syntax error at - line 1, near ". ="
16/Sep/1999 perl 5.005, patch 03 10
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
o Parsing
Better parsing in perl 5
sub foo {}
&foo
print("hello, world\n");
# perl4 prints: hello, world
# perl5 prints: syntax error
o Parsing
"if it looks like a function, it is a function" rule.
print
($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
o Parsing
String interpolation of the $#array construct differs
when braces are to used around the name.
@ = (1..3);
print "${#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
operators, operands, or output from same.
o Numerical
Formatted output and significant digits
print 7.373504 - 0, "\n";
printf "%20.18f\n", 7.373504 - 0;
# Perl4 prints:
7.375039999999996141
7.37503999999999614
16/Sep/1999 perl 5.005, patch 03 11
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# Perl5 prints:
7.373504
7.37503999999999614
o 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;
o 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...
General data type traps
Perl4-to-Perl5 traps involving most data-types, and their
usage within certain expressions and/or context.
o (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
o (Arrays)
Setting $#array lower now discards array elements,
and makes them impossible to recover.
16/Sep/1999 perl 5.005, patch 03 12
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
@a = (a,b,c,d,e);
print "Before: ",join('',@a);
$#a =1;
print ", After: ",join('',@a);
$#a =3;
print ", Recovered: ",join('',@a),"\n";
# perl4 prints: Before: abcde, After: ab, Recovered: abcd
# perl5 prints: Before: abcde, After: ab, Recovered: ab
o (Hashes)
Hashes get defined before use
local($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
o (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:
o (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).
o (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
16/Sep/1999 perl 5.005, patch 03 13
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
o (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: $_[0]\n";
}
# perl4:
# before: x after: m
# before: a after: m
# before: m after: m
# before: m after: m
# Perl5:
# before: x after: m
# Modification of a read-only value attempted at foo.pl line 12.
# before: a
o (Scalars)
The behavior is slightly different for:
print "$x", defined $x
# perl 4: 1
# perl 5: <no output, $x is not called into existence>
o (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 );
16/Sep/1999 perl 5.005, patch 03 14
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
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
o (list context)
The elements of argument lists for formats are now
evaluated in list context. This means you can
interpolate 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
o (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");
16/Sep/1999 perl 5.005, patch 03 15
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# perl4 errors: There is no caller
# perl5 prints: Got a 0
o (scalar context)
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
o (list, builtin)
sprintf() funkiness (array argument converted to
scalar array count) This test could be added to
t/op/sprintf.t
@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
printf() works fine, though:
printf STDOUT (@z);
print "\n";
# perl4 prints: foobar
# perl5 prints: foobar
Probably a bug.
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.
o Precedence
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.
16/Sep/1999 perl 5.005, patch 03 16
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
@arr = ( 'left', 'right' );
$a{shift @arr} = shift @arr;
print join( ' ', keys %a );
# perl4 prints: left
# perl5 prints: right
o 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 $m\n";
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
o Precedence
The precedence of assignment operators is now the
same as the precedence of assignment. Perl 4
mistakenly gave them the precedence of the associated
operator. So you now must parenthesize them in
expressions like
/foo/ ? ($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.
o 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);
16/Sep/1999 perl 5.005, patch 03 17
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# perl4 opens or dies
# perl5 errors: Precedence problem: open FOO should be open(FOO)
o 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
o Precedence
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 -e $foo .= "q" should parse as ((-e $foo) .=
"q"), it actually parses as (-e ($foo .= "q")). In
perl5, the precedence is as documented.
-e $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
o Precedence
In perl4, keys(), each() and values() were special
high-precedence operators that operated on a single
hash, but in perl5, they are regular named unary
operators. As documented, named unary operators have
lower precedence than the arithmetic and
concatenation operators + - ., but the perl4 variants
of these operators actually bind tighter than + - ..
Thus, for:
%foo = 1..10;
print keys %foo - 1
# perl4 prints: 4
# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
The perl4 behavior was probably more useful, if less
consistent.
General Regular Expression Traps using s///, etc.
All types of RE traps.
o Regular Expression
s'$lhs'$rhs' now does no interpolation on either
side. It used to interpolate $lhs but not $rhs.
16/Sep/1999 perl 5.005, patch 03 18
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
(And still does not match a literal '$' 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
o 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 $_ "}
# perl4 prints: blah blah blah
# perl5 prints: infinite loop blah...
o Regular Expression
Currently, if you use the m//o qualifier on a regular
expression within an anonymous sub, all closures
generated from that anonymous sub will use the
regular expression as it was compiled when it was
used the very first time in any such closure. For
instance, if you say
sub build_match {
my($left,$right) = @_;
return sub { $_[0] =~ /$left stuff $right/o; };
}
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.
This is probably a bug, and may change in future
versions of Perl.
o 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";
16/Sep/1999 perl 5.005, patch 03 19
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# perl4 prints: bcde
# perl5 prints:
o 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 section on Numerical Traps for another
example of this new feature.
o Regular Expression
s`lhs`rhs` (using backticks) is now a normal
substitution, with no backtick expansion
$string = "";
$string =~ s`^`hostname`;
print $string, "\n";
# perl4 prints: <the local hostname>
# perl5 prints: hostname
o Regular Expression
Stricter parsing of variables used in regular
expressions
s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
# perl4: compiles w/o error
# perl5: with Scalar found where operator expected ..., near "$opt$plus"
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 character
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
16/Sep/1999 perl 5.005, patch 03 20
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
o 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-Specific traps.
o (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 main'SeeYa
# perl5 prints: SIGTERM is now main::1
Use -w to catch this one
o (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
o 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
16/Sep/1999 perl 5.005, patch 03 21
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
does not.
warn STDERR "Foo!";
# perl4 prints: Foo!
# perl5 prints: String found where operator expected
OS Traps
o (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...
o (SysV)
Under SysV OSes, seek() on a file opened to append >>
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.
16/Sep/1999 perl 5.005, patch 03 22
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
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,
contexts, or whatever.
o Interpolation
@ now always interpolates an array in double-quotish
strings.
print "To: someone@somewhere.com\n";
# perl4 prints: To:someone@somewhere.com
# perl5 errors : In string, @somewhere now must be written as \@somewhere
o 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
o Interpolation
Perl now sometimes evaluates arbitrary expressions
inside braces that occur within double quotes
(usually when the opening brace is preceded by $ or
@).
@www = "buz";
$foo = "foo";
$bar = "bar";
sub foo { return "bar" };
print "|@{w.w.w}|${main'foo}|";
16/Sep/1999 perl 5.005, patch 03 23
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# perl4 prints: |@{w.w.w}|foo|
# perl5 prints: |buz|bar|
Note that you can use strict; to ward off such
trappiness under perl5.
o Interpolation
The construct "this is $$x" used to interpolate the
pid at that point, but now apparently tries to
dereference $x. $$ by itself still works fine,
however.
print "this is $$x\n";
# perl4 prints: this is XXXx (XXX is the current pid)
# perl5 prints: this is
o 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:
16/Sep/1999 perl 5.005, patch 03 24
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
# perl4 prints: Yup
# perl5 prints: Yup
# and is compatible for both versions
o 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
o Interpolation
You also have to be careful about array references.
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
o Interpolation
Similarly, watch out for:
$foo = "array";
print "\$$foo{bar}\n";
# perl4 prints: $array{bar}
# perl5 prints: $
Perl 5 is looking for $array{bar} which doesn't
exist, but perl 4 is happy just to expand $foo to
"array" by itself. Watch out for this especially in
eval's.
o Interpolation
qq() string passed to eval
eval qq(
foreach \$y (keys %\$x\) {
\$count++;
}
);
# perl4 runs this ok
# perl5 prints: Can't find string terminator ")"
16/Sep/1999 perl 5.005, patch 03 25
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
DBM Traps
General DBM traps.
o 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)
o 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.
o 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;
16/Sep/1999 perl 5.005, patch 03 26
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
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.
o 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.
16/Sep/1999 perl 5.005, patch 03 27
PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)16/Sep/1999 perl 5.005, patch 03 28
