--- code/trunk/doc/html/pcre.html 2007/08/06 15:23:29 208 +++ code/trunk/doc/html/pcre.html 2007/08/09 09:52:43 211 @@ -179,44 +179,78 @@ \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE does not support this. -

-

-The following comments apply when PCRE is running in UTF-8 mode: -

-

-1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects -are checked for validity on entry to the relevant functions. If an invalid -UTF-8 string is passed, an error return is given. In some situations, you may -already know that your strings are valid, and therefore want to skip these -checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag -at compile time or at run time, PCRE assumes that the pattern or subject it -is given (respectively) contains only valid UTF-8 codes. In this case, it does -not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to -PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program -may crash. -

+

+
+Validity of UTF-8 strings +
+

+When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects +are (by default) checked for validity on entry to the relevant functions. From +release 7.3 of PCRE, the check is according the rules of RFC 3629, which are +themselves derived from the Unicode specification. Earlier releases of PCRE +followed the rules of RFC 2279, which allows the full range of 31-bit values (0 +to 0x7FFFFFFF). The current check allows only values in the range U+0 to +U+10FFFF, excluding U+D800 to U+DFFF. +

+

+The excluded code points are the "Low Surrogate Area" of Unicode, of which the +Unicode Standard says this: "The Low Surrogate Area does not contain any +character assignments, consequently no character code charts or namelists are +provided for this area. Surrogates are reserved for use with UTF-16 and then +must be used in pairs." The code points that are encoded by UTF-16 pairs are +available as independent code points in the UTF-8 encoding. (In other words, +the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up +UTF-8.) +

+

+If an invalid UTF-8 string is passed to PCRE, an error return +(PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that +your strings are valid, and therefore want to skip these checks in order to +improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or +at run time, PCRE assumes that the pattern or subject it is given +(respectively) contains only valid UTF-8 codes. In this case, it does not +diagnose an invalid UTF-8 string. +

+

+If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what +happens depends on why the string is invalid. If the string conforms to the +"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters +in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity +test, PCRE (when in UTF-8 mode) handles strings according to the more liberal +rules of RFC 2279. However, if the string does not even conform to RFC 2279, +the result is undefined. Your program may crash. +

+

+If you want to process strings of values in the full range 0 to 0x7FFFFFFF, +encoded in a UTF-8-like manner as per the old RFC, you can set +PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this +situation, you will have to apply your own validity check. +

+
+General comments about UTF-8 mode +

-2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte +1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte UTF-8 character if the value is greater than 127.

-3. Octal numbers up to \777 are recognized, and match two-byte UTF-8 +2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 characters for values greater than \177.

-4. Repeat quantifiers apply to complete UTF-8 characters, not to individual +3. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes, for example: \x{100}{3}.

-5. The dot metacharacter matches one UTF-8 character instead of a single byte. +4. The dot metacharacter matches one UTF-8 character instead of a single byte.

-6. The escape sequence \C can be used to match a single byte in UTF-8 mode, +5. The escape sequence \C can be used to match a single byte in UTF-8 mode, but its use can lead to some strange effects. This facility is not available in the alternative matching function, pcre_dfa_exec().

-7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly +6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but the characters that PCRE recognizes as digits, spaces, or word characters remain the same set as before, all with values less than 256. This remains true even when PCRE includes Unicode @@ -225,15 +259,15 @@ must use Unicode property tests such as \p{Nd}.

-8. Similarly, characters that match the POSIX named character classes are all +7. Similarly, characters that match the POSIX named character classes are all low-valued characters.

-9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes +8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode characters.

-10. Case-insensitive matching applies only to characters whose values are less +9. Case-insensitive matching applies only to characters whose values are less than 128, unless PCRE is built with Unicode property support. Even when Unicode property support is available, PCRE still uses its own character tables when checking the case of low-valued characters, so as not to degrade performance. @@ -259,7 +293,7 @@


REVISION

-Last updated: 06 August 2007 +Last updated: 09 August 2007
Copyright © 1997-2007 University of Cambridge.