perltrap(1)
NAME
perltrap - Perl traps for the unwary
DESCRIPTION
The biggest trap of all is forgetting to "use warnings" or use the -w
switch; see perllexwarn and perlrun. The second biggest trap is not
making your entire program runnable under "use strict". The third big-
gest trap is not reading the list of changes in this version of Perl;
see perldelta.
Awk Traps
Accustomed awk users should take special note of the following:
o A Perl program executes only once, not once for each input line.
You can do an implicit loop with "-n" or "-p".
o The English module, loaded via
use English;
allows you to refer to special variables (like $/) with names (like
$RS), as though they were in awk; see perlvar for details.
o Semicolons are required after all simple statements in Perl (except
at the end of a block). Newline is not a statement delimiter.
o Curly brackets are required on "if"s and "while"s.
o Variables begin with "$", "@" 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 com-
parisons.
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 perlvar.
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 oper-
ator, 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 tok-
enizer 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 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 $;
o You cannot set $RS to a pattern, only a string.
o When in doubt, run the awk construct through a2p and see what it
gives you.
C/C++ Traps
Cerebral C and C++ programmers should take note of the following:
o Curly brackets are required on "if"'s and "while"'s.
o You must use "elsif" rather than "else if".
o The "break" and "continue" keywords from C become in Perl "last"
and "next", respectively. Unlike in C, these do not work within a
"do { } while" construct. See "Loop Control" in perlsyn.
o There's no switch statement. (But it's easy to build one on the
fly, see "Basic BLOCKs and Switch Statements" in perlsyn)
o Variables begin with "$", "@" or "%" in Perl.
o Comments begin with "#", not "/*" or "//". Perl may interpret
C/C++ comments as division operators, unterminated regular expres-
sions or the defined-or operator.
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 $0.
o System calls such as link(), unlink(), rename(), etc. return
nonzero for success, not 0. (system(), however, returns zero for
success.)
o Signal handlers deal with signal names, not numbers. Use "kill -l"
to find their names on your system.
Sed Traps
Seasoned sed programmers should take note of the following:
o A Perl program executes only once, not once for each input line.
You can do an implicit loop with "-n" or "-p".
o Backreferences in substitutions use "$" 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:
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 pat-
terns.
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.
o The shell's "test" uses "=", "!=", "<" etc for string comparisons
and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the
reverse of Perl, which uses "eq", "ne", "lt" for string compar-
isons, and "==", "!=" "<" etc for numeric comparisons.
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 perldata 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 func-
tion 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()). (Unless prototyped, user-defined sub-
routines can only be list operators, never unary ones.) See perlop
and perlsub.
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 read-
line 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!
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 con-
trol on.
o Use "my()" for local variables whenever you can get away with it
(but see perlform for where you can't). Using "local()" actually
gives a local value to a global variable, which leaves you open to
unforeseen side-effects of dynamic scoping.
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 fea-
ture 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/declara-
tions.
Precedence Traps
Traps related to the precedence of parsing, evaluation, and execu-
tion 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 imple-
mentations.
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.
* Symbols starting with "_" no longer forced into main
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
* 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 pack-
ages 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 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 $:.
* 2nd and 3rd args to "splice()" are now in scalar context
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
* Can't do "goto" into a block that is optimized away
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
* 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.
$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
* "while/if BLOCK BLOCK" gone
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 {"
* "**" binds tighter than unary minus
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
* "foreach" changed when iterating over a list
The meaning of "foreach{}" has changed slightly when it is iterat-
ing 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 $_.)
* "split" with no args behavior changed
"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
* -e behavior fixed
Perl 4 would ignore any text which was attached to an -e switch,
always taking the code snippet from the following arg. Addition-
ally, 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.
* "push" returns number of elements in resulting list
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
* Some error messages differ
Some error messages will be different.
* "split()" honors subroutine args
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 argu-
ments.
* Bugs removed
Some bugs may have been inadvertently removed. :-)
Parsing Traps
Perl4-to-Perl5 traps from having to do with parsing.
* Space between . and = triggers syntax error
Note the space between . and =
$string . = "more string";
print $string;
# perl4 prints: more string
# perl5 prints: syntax error at - line 1, near ". ="
* Better parsing in perl 5
Better parsing in perl 5
sub foo {}
&foo
print("hello, world\n");
# perl4 prints: hello, world
# perl5 prints: syntax error
* Function parsing
"if it looks like a function, it is a function" rule.
print
($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
* String interpolation of $#array differs
String interpolation of the $#array construct differs when braces
are to used around the name.
@a = (1..3);
print "${#a}";
# perl4 prints: 2
# perl5 fails with syntax error
@ = (1..3);
print "$#{a}";
# perl4 prints: {a}
# perl5 prints: 2
* Perl guesses on "map", "grep" followed by "{" if it starts BLOCK or
hash ref
When perl sees "map {" (or "grep {"), it has to guess whether the
"{" starts a BLOCK or a hash reference. If it guesses wrong, it
will report a syntax error near the "}" and the missing (or unex-
pected) comma.
Use unary "+" before "{" on a hash reference, and unary "+" applied
to the first thing in a BLOCK (after "{"), for perl to guess right
all the time. (See "map" in perlfunc.)
Numerical Traps
Perl4-to-Perl5 traps having to do with numerical operators, operands,
or output from same.
* 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;
# Perl4 prints:
7.3750399999999996141
7.375039999999999614
# Perl5 prints:
7.373504
7.375039999999999614
Notice how the first result looks better in Perl 5.
Your results may vary, since your floating point formatting rou-
tines and even floating point format may be slightly different.
* 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 in version 5.003_04. But always be
wary when using large integers. If in doubt:
use Math::BigInt;
* 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 a null, instead of 0
$p = ($test == 1);
print $p,"\n";
# perl4 prints: 0
# perl5 prints:
Also see "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 bitstrings so long as the program con-
tained a call to the "vec()" function. perl5 treats the string op-
erands as bitstrings. (See "Bitwise String Operators" in perlop
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.
* 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
* Setting $#array lower now discards array elements
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);
$#a =3;
print ", Recovered: ",join('',@a),"\n";
# perl4 prints: Before: abcde, After: ab, Recovered: abcd
# perl5 prints: Before: abcde, After: ab, Recovered: ab
* Hashes get defined before use
Hashes get defined before use
local($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).
* 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
@a = ("This is Perl 4");
*b = *a;
local(@a);
print @b,"\n";
# perl4 prints: This is Perl 4
# perl5 prints:
* Assigning "undef" to glob
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
* Changes in unary negation (of strings)
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
* Modifying of constants prohibited
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
* "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>
* 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
* 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 = ("foo","bar","baz");
format STDOUT=
@<<<<< @||||| @>>>>>
@fmt;
.
write;
# perl4 errors: Please use commas to separate fields in file
# perl5 prints: foo bar baz
* "caller()" returns false value in scalar context if no caller present
The "caller()" function now returns a false value in a scalar con-
text 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
* 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 = $x\n";
# Perl4 prints: x = c # Thinks list context interpolates list
# Perl5 prints: x = 3 # Knows scalar uses length of list
* "sprintf()" prototyped as "($;@)"
"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 inconsis-
tencies that made the behavior differ from what was documented.
* LHS vs. RHS of any assignment operator
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;
print join( ' ', keys %a );
# perl4 prints: left
# perl5 prints: right
* Semantic errors introduced due to 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 $m\n";
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
* Precedence of assignment operators same as the precedence of assign-
ment
The precedence of assignment operators is now the same as the
precedence of assignment. Perl 4 mistakenly gave them the prece-
dence of the associated operator. So you now must parenthesize
them in expressions like
/foo/ ? ($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.
* "open" requires parentheses around filehandle
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 over $:: gone
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 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 "-e $foo .= "q"" should parse as "((-e
$foo) .= "q")", it actually parses as "(-e ($foo .= "q"))". In
perl5, the precedence is as documented.
-e $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
* "keys", "each", "values" are regular named unary operators
In perl4, keys(), each() and values() were special high-precedence
operators that operated on a single hash, but in perl5, they are
regular named unary operators. As documented, named unary opera-
tors have lower precedence than the arithmetic and concatenation
operators "+ - .", but the perl4 variants of these operators actu-
ally bind tighter than "+ - .". Thus, for:
%foo = 1..10;
print keys %foo - 1
# perl4 prints: 4
# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
The perl4 behavior was probably more useful, if less consistent.
General Regular Expression Traps using s///, etc.
All types of RE traps.
* "s'$lhs'$rhs'" interpolates on either side
"s'$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
* "m//g" attaches its state to the searched string
"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 $_ "}
# perl4 prints: Got blah Got blah Got blah Got blah
# perl5 prints: infinite loop blah...
* "m//o" used within an anonymous sub
Currently, if you use the "m//o" qualifier on a regular expression
within an anonymous sub, all closures generated from that anony-
mous sub will use the regular expression as it was compiled when
it was used the very first time in any such closure. For
instance, if you say
sub build_match {
my($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 first time that build_match()
was called, not as they are in the current call.
* $+ isn't set to whole match
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:
* 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 "Numerical Traps" for another example of this new fea-
ture.
* "s`lhs`rhs`" is now a normal substitution
"s`lhs`rhs`" (using backticks) is now a normal substitution, with
no backtick expansion
$string = "";
$string =~ s`^`hostname`;
print $string, "\n";
# perl4 prints: <the local hostname>
# perl5 prints: hostname
* Stricter parsing of variables in regular expressions
Stricter parsing of variables used in regular expressions
s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
# perl4: compiles w/o error
# perl5: with Scalar found where operator expected ..., near "$opt$plus"
an added component of this example, apparently from the same
script, is the actual value of the s'd string after the substitu-
tion. "[$opt]" is a character class in perl4 and an array sub-
script in perl5
$grpc = 'a';
$opt = 'r';
$_ = 'bar';
s/^([^$grpc]*$grpc[$opt]?)/foo/;
print;
# perl4 prints: foo
# perl5 prints: foobar
* "m?x?" matches only once
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
* Failed matches don't reset the match variables
Unlike in Ruby, failed matches in Perl do not reset the match
variables ($1, $2, ..., $`, ...).
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.
* 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 now main'SeeYa
# perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
Use -w to catch this one
* 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 (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 spec-
ify 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 resets signal handler correctly
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 cor-
rectly. 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 "seek()" appends correctly
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.
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.
* "@" always interpolates an array in double-quotish strings
@ 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
* Double-quoted strings may no longer end with an unescaped $
Double-quoted strings may no longer end with an unescaped $.
$foo = "foo$";
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 $bar
* Arbitrary expressions are evaluated inside braces within double
quotes
Perl now sometimes evaluates arbitrary expressions inside braces
that occur within double quotes (usually when the opening brace is
preceded by "$" 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.
* $$x now tries to dereference $x
The construct "this is $$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
* Creation of hashes on the fly with "eval "EXPR"" requires protection
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 pro-
tected, 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
* 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
* Array and hash brackets during interpolation
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
Perl 5 is expecting to find an index or key name following the
respective brackets, as well as an ending bracket of the appropri-
ate type. In order to mimic the behavior of Perl 4, you must
escape the bracket like so.
print "$foo\[";
print "$foo\{";
* Interpolation of "\$$foo{bar}"
Similarly, watch out for: "\$$foo{bar}"
$foo = "baz";
print "\$$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.
* "qq()" string passed to "eval" will not find string terminator
"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.
* Perl5 must have been linked with same dbm/ndbm as the default for
"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 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 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 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.
perl v5.8.8 2006-06-14 PERLTRAP(1)
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