the characters -~ in the character class

Unless the hyphen is the very first character, it signifies a range. In this case, it's the range ' ' (space) through '~' (tilde) - which is essentially the entire range of printable ASCII characters.

So what this test is saying: is there a "line" of "text" - where "line" is defined by the ^ and $ anchors, and "text" is defined as "all the printable ASCII characters, plus the selected whitespace characters newline, carriage return, tab, and space."

> Unless the hyphen is the very first character, it signifies a range.

or the very last.

  DB<1> p '-' =~ /[a-]/
1
  DB<2> p '-' =~ /[a- ]/
Invalid [] range "a- " in re ..
[download]

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

A ^ before [] means negation.

That's a misunderstanding. The ^ at the start means match the start of the string. A ^ inside [] would be negation.

> I ran across it while seeking an alternative to -T to check for a text file

FYI, it seems the OP ran across this function at this stackoverflow question.

Perl can help you understand this kind of thing on your own. I find YAPE::Regex::Explain handy for explaining "simple" regexes. Note that there is no support for regex syntax added after Perl version 5.6.

Win8 Strawberry 5.30.3.1 (64)  Thu 07/22/2021 20:17:20
C:\@Work\Perl\monks
>perl -wMstrict -MYAPE::Regex::Explain
-e "print YAPE::Regex::Explain->new(qr/^[\r\n\t -~]*$/s)->explain;"
The regular expression:

(?s-imx:^[\r\n\t -~]*$)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?s-imx:                 group, but do not capture (with . matching
                         \n) (case-sensitive) (with ^ and $ matching
                         normally) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  [\r\n\t -~]*             any character of: '\r' (carriage return),
                           '\n' (newline), '\t' (tab), ' ' to '~' (0
                           or more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

No, the caret only negates the character set if it appears immediately after the left square bracket. Outside square brackets, it matches the beginning of the string or immediately after a newline. Also: the  -~ sequence inside square brackets means 'any character between space and tilde, inclusive.'

So in words, the regular expression specifies 'all characters in the first 4096 (or end-of-file, whichever comes first) are "\r", "\n", "\t", or characters in the range space to tilde, inclusive, in your machine's native encoding'.

Off-topic comments:

• The -T operator will tell you if your file is ASCII/UTF-8. Just say return -T $_[0];.
• The special file handle _ (i.e. underscore) means "whatever file was last tested" under any recent Perl, and can be faster because it makes use of the same stat() structure.
• The three-argument form of open() is preferred because it handles file names with strange characters better. In your case it would be open FH, "<", $_[0]
• You should probably ensure that your open() succeeded. Something like open FH, "<", $_[0] or die "Failed to open $_[0]: $!"; is the usual idiom.
• People usually use lexical file handles rather than bareword file handles these days because they get closed automatically when they go out of scope (say, if you throw an unexpected exception). In your code that would look like open my $fh, "<", $_[0] ....

Outside square brackets, it [caret] matches the beginning of the string or immediately after a newline.

By default, ^ (caret) outside a character class matches only at the beginning of a string, exactly as \A does. Caret (outside a character class) also matches immediately after an embedded newline if the /m modifier is asserted. See Modifiers in perlre.

---

Give a man a fish:  <%-{-{-{-<

It could give a false positive if every byte of a 'binary' file happened to contain the code of an ASCII character. (Can you tolerate an extremely rare error?)

I know only one binary file that intentionally contains only ASCII characters: The EICAR test file. Here it is, in its full glory:

X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*
[download]

It is a DOS executable that just prints the embedded text "EICAR-STANDARD-ANTIVIRUS-TEST-FILE!" and then exits.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

Clever. I was thinking of a binary data file (Perhaps a sequence of readings from a 7-bit A/D converter).
Bill

impressive! and another form of golf is born.