Perl::Critic::Policy::ValuesAndExpressions::ProhibitUnknownBackslash - don't use undefined backslash forms
This policy is part of the Perl::Critic::Pulp add-on. It checks for unknown backslash escapes like
Perl::Critic::Pulp
print "\*.c"; # bad
This is harmless, assuming the intention is a literal "*" (which it gives), but unnecessary, and on that basis this policy is under the cosmetic theme (see "POLICY THEMES" in Perl::Critic). Sometimes it can be a misunderstanding or a typo though, for instance a backslashed newline is a newline, but perhaps you thought it meant a continuation.
cosmetic
print "this\ # bad is a newline";
Perl already warns about unknown escaped alphanumerics like \v under perl -w or use warnings (see "Unrecognized escape \%c passed through" in perldiag).
\v
perl -w
use warnings
print "\v"; # bad, and provokes Perl warning
This policy extends to report on any unknown escape, with options below to vary the strictness and to check single-quote strings too if desired.
Control characters \cX are checked and only the conventional A-Z a-z @ [ \ ] ^ _ ? are considered known.
\cX
print "\c*"; # bad
Perl accepts any \c and does an upcase xor 0x40, so \c* is letter "j", at least on an ASCII system. But that's obscure and likely to be a typo or error.
\c
\c*
For reference, \c\ is the ASCII FS "file separator" and the second backslash is not an escape, except for a closing quote character, which it does escape (basically because Perl scans for a closing quote before considering interpolations). Thus,
\c\
print " \c\ "; # ok, control-\ FS print " \c\" "; # bad, control-" is unknown print qq[ \c\] ]; # ok, control-] GS
A backslashed colon, bracket, brace or dash is allowed after an interpolated variable or element to stop interpolation at that point.
print "$foo\::bar"; # ok, $foo print "@foo\::"; # ok, @foo print "$foo[0]\[1]"; # ok, is $foo[0] print "$esc\[1m"; # ok print "$foo\{k}"; # ok print "$foo\{k}"; # ok print "$foo{k}\[0]"; # ok, is $foo{k} print "@foo\{1,2}"; # ok, is @foo print "$foo\->[0]"; # ok, is $foo print "$foo\->{zz}"; # ok
A single backslash like "\::" is enough for the colon case, but backslashing the second too as "\:\:" is quite common and is allowed.
"\::"
"\:\:"
print "$#foo\:\:bar"; # ok
Only a ->[] or ->{} needs a \- to stop interpolation. Other cases such as an apparent method call or arrowed coderef call don't interpolate and the backslash is treated as unknown since unnecessary.
->[]
->{}
\-
print "$coderef\->(123)"; # bad, unnecessary print "Usage: $class\->foo()"; # bad, unnecessary
For reference, the alternative in all the above is to write {} braces around the variable or element to delimit from anything following. Doing so may be clearer than backslashing,
{}
print "${foo}::bar"; # alternatives print "@{foo}::bar"; print "$#{foo}th"; print "${foo[0]}[1]"; # array element $foo[0]
The full horror story of backslashing interpolations can be found in "Gory details of parsing quoted constructs" in perlop.
Octal escapes above \400 to \777 for wide chars 256 to 511 are new in Perl 5.6. They're considered unknown in 5.005 and earlier (where they end up chopped to 8-bits 0 to 255). Currently if there's no use etc Perl version then it's presumed a high octal is intentional and is allowed.
\400
\777
use
print "\400"; # ok use 5.006; print "\777"; # ok use 5.005; print "\777"; # bad in 5.005 and earlier
Named chars \N{SOME THING} are added by charnames (new in Perl 5.6) and are treated as known if there's a use charnames in the lexical scope.
\N{SOME THING}
use charnames
{ use charnames ':full'; print "\N{APOSTROPHE}"; # ok } print "\N{COLON}"; # bad without charnames
A \N without charnames is a compile error in Perl 5.6 or higher so is normally seen immediately anyway.
\N
charnames
In the violation messages a non-ascii or non-graphical escaped char is shown as hex like \{0x263A}, to ensure the message is printable and unambiguous.
\{0x263A}
Interpolated $foo or @{expr} variables and expressions are parsed like Perl does, so backslashes for refs within are ok, in particular tricks like ${\scalar ...} are fine (see "How do I expand function calls in a string?" in perlfaq4).
$foo
@{expr}
${\scalar ...}
print "this ${\(some()+thing())}"; # ok
As always, if you're not interested in any of this then you can disable ProhibitUnknownBackslash from your .perlcriticrc in the usual way (see "CONFIGURATION" in Perl::Critic),
ProhibitUnknownBackslash
[-ValuesAndExpressions::ProhibitUnknownBackslash]
double
heredoc
double applies to double-quote strings "", qq{}, qx{}, etc. heredoc applies to interpolated here-documents <<HERE etc. The possible values are
""
qq{}
qx{}
<<HERE
none don't report anything alnum report unknown alphanumerics, like Perl's warning quotemeta report anything quotemeta() doesn't escape all report all unknowns
"alnum" does no more than compiling with perl -w, but might be good for checking code you don't want to run.
"quotemeta" reports escapes not produced by quotemeta(). For example quotemeta escapes a *, so \* is not reported, but it doesn't escape an underscore _, so \_ is reported. The effect is to prohibit a few more escapes than "alnum". One use is to check code generated by other code where you've used quotemeta to produce double-quoted strings and thus may have escaping which is unnecessary but works fine.
quotemeta()
quotemeta
*
\*
_
\_
single
single applies to single-quote strings '', q{}, qx'', etc. The possible values are as above, though only "all" or "none" make much sense.
''
q{}
qx''
none don't report anything all report all unknowns
The default is "none" because literal backslashes in single-quotes are usually both what you want and quite convenient. Setting "all" effectively means you must write backslashes as \\.
\\
print 'c:\my\msdos\filename'; # bad under "single=all" print 'c:\\my\\msdos\\filename'; # ok
Doubled backslashing like this is correct, and can emphasize that you really did want a backslash, but it's tedious and not easy on the eye and so is left only as an option.
For reference, single-quote here-documents <<'HERE' don't have any backslash escapes and so are not considered by this policy. qx{} command backticks are double-quote but as qx'' is single-quote and in each case treated under the corresponding single/double option.
<<'HERE'
Perl::Critic::Pulp, Perl::Critic
"Quote and Quote-like Operators" in perlop
http://user42.tuxfamily.org/perl-critic-pulp/index.html
Copyright 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016 Kevin Ryde
Perl-Critic-Pulp is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.
Perl-Critic-Pulp is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Perl-Critic-Pulp. If not, see <http://www.gnu.org/licenses>.
To install Perl::Critic::Pulp, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Perl::Critic::Pulp
CPAN shell
perl -MCPAN -e shell install Perl::Critic::Pulp
For more information on module installation, please visit the detailed CPAN module installation guide.