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Re^5: Unescaped left brace in regex is passed through in regex

by AnomalousMonk (Archbishop)
on Jun 07, 2022 at 21:24 UTC ( [id://11144486]=note: print w/replies, xml ) Need Help??


in reply to Re^4: Unescaped left brace in regex is passed through in regex
in thread Unescaped left brace in regex is passed through in regex

I'm not the best one to answer questions about the Perl debugger, but I can answer some more general questions.

Q0) ... we begin the game with a string that has 2 backslashes ... the output differs from the original string by having one fewer backslash.
Due to the way the single-quote string constructor handles backslashes (escapes), the \\ will in this case compile to a single literal backslash. See Quote and Quote-like Operators and the discussion of q/STRING/ in Quote-Like Operators.
Q1) What makes you think this is a left-curly brace?
I'm not quite sure what "this" refers to, but do you dispute that there is a left-curly (and a right-curly) in the \x{A3f4} string? What else would you call it/them?
Q2: What is \x{value} called? ...
I would call it (or in this case \x{A3f4}) "the string compiled from '\\x{A3f4}'". The \x part has nothing to do with the /x or /xx regex modifiers.


Give a man a fish:  <%-{-{-{-<

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Re^6: Unescaped left brace in regex is passed through in regex
by Aldebaran (Curate) on Jun 08, 2022 at 00:20 UTC
    Due to the way the single-quote string constructor handles backslashes (escapes), the \\ will in this case compile to a single literal backslash. See Quote and Quote-like Operators and the discussion of q/STRING/ in Quote-Like Operators.

    Thanks for your comment, AM. Your link is a good read and worth reposting. I thought that the collapsing of backslashes was done by the OS in resolving paths. I was unaware that perl did it.

    do you dispute that there is a left-curly (and a right-curly) in the \x{A3f4} string? What else would you call it/them?

    I do not dispute that, so this string itself never represents a left curly brace, rather it has a left curly brace in it.

    I would call it (or in this case \x{A3f4}) "the string compiled from '\\x{A3f4}'"

    Ok. From the above source we have:

    \x{263A} [1,8] hex char (example shown: SMILEY) \x{ 263A } Same, but shows optional blanks inside and adjoining the braces \x1b [2,8] restricted range hex char (example: ESC)

    So, I think "aha, it's a hex representation", but then I can't get there with the REPL:

    DB<1> $str2='\\x{263}' + DB<2> p $str2 + \x{263} DB<3> p hex $str2 + 0 DB<4> print hex $str2 + 0

    I would expect to see a smiley face rather than zero. This is a head-scratcher:

    DB<6> $str3='\\\\\\\x{aF}' + DB<7> p $str3 + \\\\x{aF} DB<8> p hex $str3 + 0 DB<9> print hex $str3 + 0

    $str3 goes from 7 to 4 backslashes when compiled(?). But I get zero for a hex value no matter what I try:

    DB<10> $str4='\x{aF}' + DB<11> p $str4 + \x{aF} DB<12> print hex $str4 + 0 DB<13> print hex 'aF' + 175

    How do I tease 175 out of $str4?

    The \x part has nothing to do with the /x or /xx regex modifiers.

    That part is clearer now. I have that backslash/forwardslash disphoria going on now where I can hardly see the difference and it looks like a toothpick war. I get the occasional billiken that I read or write the wrong way.

      > So, I think "aha, it's a hex representation", but then I can't get there with the REPL:

      you are still confusing interpolation (double-quotes) from literal strings (single-quotes)

      DB<28> p $str1 = "\x{41}" # interpolation A DB<29> p $str2 = '\x{41}' # literal \x{41} DB<30> p $str2 = '\\x{41}' # literal but escaping escaping \ \x{41}

      now, the double escape in line 30 is playing safe, because there is a difference between \\' and \'

      BUT this

      \x{ 263A } Same, but shows optional blanks inside and adjoining the braces

      doesn't work for me! (oO ???)

      DB<31> p " \x{ 41 } " ^@

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

      Some random responses...

      do you dispute that there is a left-curly (and a right-curly) in the \x{A3f4} string? What else would you call it/them?
      I do not dispute that, so this string itself never represents a left curly brace, rather it has a left curly brace in it.

      Oh, so you were thinking that "\x{A3f4}" when compiled double-quotishy into a string and then printed should print a left-curly! I follow you a little better now. My terminal is not configured for Unicode (as I assume this character to be) right now, so I cannot confirm what it will print, and I'm reluctant to launch myself into Unicode-land on-line to find out. However, I agree that the escape sequence \x{A3f4} when compiled double-quotishly (e.g., "ab\x{A3f4}cd") will compile to some character. But the single-quote-compiled string '\x{A3f4}' will always be literally \x{A3f4} and nothing else.

      It's important to understand how backslashes (escapes) are compiled in single- and double-quoted strings. Consider the following:

      Win8 Strawberry 5.8.9.5 (32) Tue 06/07/2022 12:17:53 C:\@Work\Perl\monks >perl use strict; use warnings; print '-\-\\-\\\-\\\\-\\\\\-\\\\\\-\\\\\\\-\\\\\\\\-'; ^Z -\-\-\\-\\-\\\-\\\-\\\\-\\\\-
      Why do '\\\\\\\' and '\\\\\\\\' (7 and 8 backslashes, respectively) both compile to and print as four backslashes? How would this be different if compiled as a double-quoted string?

      DB<1> $str2='\\x{263}'

      This compiles to (and prints) the literal string \x{263} or literal-backslash, literal-lowercase-x, literal-left-curly, literal-2, literal-6, literal-3, literal-right-curly. The hex built-in cannot interpret a string in this format (and so returns zero (update: and a warning)), but can in "proper" format:

      Win8 Strawberry 5.8.9.5 (32) Tue 06/07/2022 22:09:02 C:\@Work\Perl\monks >perl use strict; use warnings; my $h1 = 'A3f4'; my $h2 = 'xA3f4'; print hex 'A3f4', "\n"; print hex $h1, "\n"; print hex 'xA3f4', "\n"; print hex $h2, "\n"; print hex '\xA3f4', "\n"; print hex '\x{A3f4}', "\n"; ^Z 41972 41972 41972 41972 Illegal hexadecimal digit '\' ignored at - line 13. 0 Illegal hexadecimal digit '\' ignored at - line 14. 0

          DB<10> $str4='\x{aF}'
      ...
      How do I tease 175 out of $str4?

      We know that \x{aF} will not be interpreted by hex as a hex number. One way to extract the hex substring:

      Win8 Strawberry 5.8.9.5 (32) Tue 06/07/2022 22:25:09 C:\@Work\Perl\monks >perl use strict; use warnings; my $str = '\x{aF}'; $str =~ m{ \A \\ x \{ ([[:xdigit:]]+) \} \z }xms; my $hex_digits = $1; print ">$hex_digits< \n"; my $hex_number_in_decimal = hex $hex_digits; print "$hex_number_in_decimal \n"; ^Z >aF< 175

      Update: Another approach:

      Win8 Strawberry 5.8.9.5 (32) Sat 06/11/2022 15:18:47 C:\@Work\Perl\monks >perl use strict; use warnings; my $str = '\x{aF}'; my ($hex_digits) = $str =~ m{ [[:xdigit:]]+ }xmsg; my $hex_number_in_decimal = hex $hex_digits; print "'$hex_digits' == $hex_number_in_decimal decimal \n"; ^Z 'aF' == 175 decimal
      This approach can be useful when a string or record has been "validated" as to its structure and you know that certain substrings or fields are unambiguously present: these substrings/fields can then be easily and quickly extracted. Note the /g modifier on the m// match.


      Give a man a fish:  <%-{-{-{-<

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