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third_party/sqlite/sqlite-src-3080704/ext/icu/README.txt

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-
-This directory contains source code for the SQLite "ICU" extension, an
-integration of the "International Components for Unicode" library with
-SQLite. Documentation follows.
-
-    1. Features
-    
-        1.1  SQL Scalars upper() and lower()
-        1.2  Unicode Aware LIKE Operator
-        1.3  ICU Collation Sequences
-        1.4  SQL REGEXP Operator
-    
-    2. Compilation and Usage
-    
-    3. Bugs, Problems and Security Issues
-    
-        3.1  The "case_sensitive_like" Pragma
-        3.2  The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro
-        3.3  Collation Sequence Security Issue
-
-
-1. FEATURES
-
-  1.1  SQL Scalars upper() and lower()
-
-    SQLite's built-in implementations of these two functions only 
-    provide case mapping for the 26 letters used in the English
-    language. The ICU based functions provided by this extension
-    provide case mapping, where defined, for the full range of 
-    unicode characters.
-
-    ICU provides two types of case mapping, "general" case mapping and
-    "language specific". Refer to ICU documentation for the differences
-    between the two. Specifically:
-
-       http://www.icu-project.org/userguide/caseMappings.html
-       http://www.icu-project.org/userguide/posix.html#case_mappings
-
-    To utilise "general" case mapping, the upper() or lower() scalar 
-    functions are invoked with one argument:
-
-        upper('ABC') -> 'abc'
-        lower('abc') -> 'ABC'
-
-    To access ICU "language specific" case mapping, upper() or lower()
-    should be invoked with two arguments. The second argument is the name
-    of the locale to use. Passing an empty string ("") or SQL NULL value
-    as the second argument is the same as invoking the 1 argument version
-    of upper() or lower():
-
-        lower('I', 'en_us') -> 'i'
-        lower('I', 'tr_tr') -> 'ı' (small dotless i)
-
-  1.2  Unicode Aware LIKE Operator
-
-    Similarly to the upper() and lower() functions, the built-in SQLite LIKE
-    operator understands case equivalence for the 26 letters of the English
-    language alphabet. The implementation of LIKE included in this
-    extension uses the ICU function u_foldCase() to provide case
-    independent comparisons for the full range of unicode characters.  
-
-    The U_FOLD_CASE_DEFAULT flag is passed to u_foldCase(), meaning the
-    dotless 'I' character used in the Turkish language is considered
-    to be in the same equivalence class as the dotted 'I' character
-    used by many languages (including English).
-
-  1.3  ICU Collation Sequences
-
-    A special SQL scalar function, icu_load_collation() is provided that 
-    may be used to register ICU collation sequences with SQLite. It
-    is always called with exactly two arguments, the ICU locale 
-    identifying the collation sequence to ICU, and the name of the
-    SQLite collation sequence to create. For example, to create an
-    SQLite collation sequence named "turkish" using Turkish language
-    sorting rules, the SQL statement:
-
-        SELECT icu_load_collation('tr_TR', 'turkish');
-
-    Or, for Australian English:
-
-        SELECT icu_load_collation('en_AU', 'australian');
-
-    The identifiers "turkish" and "australian" may then be used
-    as collation sequence identifiers in SQL statements:
-
-        CREATE TABLE aust_turkish_penpals(
-          australian_penpal_name TEXT COLLATE australian,
-          turkish_penpal_name    TEXT COLLATE turkish
-        );
-  
-  1.4 SQL REGEXP Operator
-
-    This extension provides an implementation of the SQL binary
-    comparision operator "REGEXP", based on the regular expression functions
-    provided by the ICU library. The syntax of the operator is as described
-    in SQLite documentation:
-
-        <string> REGEXP <re-pattern>
-
-    This extension uses the ICU defaults for regular expression matching
-    behavior. Specifically, this means that:
-
-        * Matching is case-sensitive,
-        * Regular expression comments are not allowed within patterns, and
-        * The '^' and '$' characters match the beginning and end of the
-          <string> argument, not the beginning and end of lines within
-          the <string> argument.
-
-    Even more specifically, the value passed to the "flags" parameter
-    of ICU C function uregex_open() is 0.
-
-
-2  COMPILATION AND USAGE
-
-  The easiest way to compile and use the ICU extension is to build
-  and use it as a dynamically loadable SQLite extension. To do this
-  using gcc on *nix:
-
-    gcc -shared icu.c `icu-config --ldflags` -o libSqliteIcu.so
-
-  You may need to add "-I" flags so that gcc can find sqlite3ext.h
-  and sqlite3.h. The resulting shared lib, libSqliteIcu.so, may be
-  loaded into sqlite in the same way as any other dynamically loadable
-  extension.
-
-
-3 BUGS, PROBLEMS AND SECURITY ISSUES
-
-  3.1 The "case_sensitive_like" Pragma
-
-    This extension does not work well with the "case_sensitive_like"
-    pragma. If this pragma is used before the ICU extension is loaded,
-    then the pragma has no effect. If the pragma is used after the ICU
-    extension is loaded, then SQLite ignores the ICU implementation and
-    always uses the built-in LIKE operator.
-
-    The ICU extension LIKE operator is always case insensitive.
-
-  3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro
-
-    Passing very long patterns to the built-in SQLite LIKE operator can
-    cause excessive CPU usage. To curb this problem, SQLite defines the
-    SQLITE_MAX_LIKE_PATTERN_LENGTH macro as the maximum length of a
-    pattern in bytes (irrespective of encoding). The default value is
-    defined in internal header file "limits.h".
-    
-    The ICU extension LIKE implementation suffers from the same 
-    problem and uses the same solution. However, since the ICU extension
-    code does not include the SQLite file "limits.h", modifying
-    the default value therein does not affect the ICU extension.
-    The default value of SQLITE_MAX_LIKE_PATTERN_LENGTH used by
-    the ICU extension LIKE operator is 50000, defined in source 
-    file "icu.c".
-
-  3.3 Collation Sequence Security Issue
-
-    Internally, SQLite assumes that indices stored in database files
-    are sorted according to the collation sequence indicated by the
-    SQL schema. Changing the definition of a collation sequence after
-    an index has been built is therefore equivalent to database
-    corruption. The SQLite library is not very well tested under
-    these conditions, and may contain potential buffer overruns
-    or other programming errors that could be exploited by a malicious
-    programmer.
-
-    If the ICU extension is used in an environment where potentially
-    malicious users may execute arbitrary SQL (i.e. gears), they
-    should be prevented from invoking the icu_load_collation() function,
-    possibly using the authorisation callback.