Java \ cx regular expression (escape characters)

Question

Javadoc for java.util.regex.Patternsays \cxrepresents the control character corresponding to x. So I thought I would Pattern.compile()reject \c, followed by any character other than [@-_], but it is not!

As @tchrist commented on one of the answers to What is a regex for control characters? , the range is not checked at all. I tested a couple of characters from the higher blocks, as well as the astral planes, it looks like it just flips the 7th LSB of the code point value.

So is it a Javadoc error or an implementation error, or am I not understanding something? Is \cxJava syntax developed or supported by other regular expression engines, especially Perl? How is it processed there?

Answer 1

All versions of Perl behave the same for the following screens:

• If followed \cby an ASCII capital letter or one of @[\]^_?,

  chr(ord($char) ^ 0x40)

  This provides full coverage of all ASCII control characters ( 0x00.. 0x1F, 0x7F).

  \c@ === \x00
  \cA === \x01
  ...
  \cZ === \x1A
  \c[ === \x1B
  \c\ === \x1C   # Sometimes \c\\ is needed.
  \c] === \x1D
  \c^ === \x1E
  \c_ === \x1F
  \c? === \x7F
  

• When \cfollowed by a lowercase ASCII,

  chr(ord($char) ^ 0x60)

  This makes the escape register insensitive.

  \ca === \cA === \x01
  ...
  \cz === \cZ === \x1A
  

---

, Perl 5.20.

• & GE; 5,20,

  • \c , ASCII !"#$%&'()*+,-./:;<=>{|}~,

    chr(ord($char) ^ 0x40), (is more clearly written simply as).

  • \c ASCII (0x00.. 0x1F, 0x7F) ASCII (& ge; 0x80),

    Character following "\c" must be printable ASCII.

• < 5,20,

  • \c , ASCII-, !"#$%&'()*+,-./:;<=>{|}~ ASCII (0x00.. 0x1F, 0x7F),

    chr(ord($char) ^ 0x40)

  • \c & ge; 0x100,

    (chr(ord(substr(encode_utf8($char, 0, 1)) ^ 0x40) . encode_utf8($char, 1)).

  • \c 0x80.. 0xFF,

    chr(ord($char) ^ 0x40), , & ge; 0x100.