1 2This directory contains source code for the SQLite "ICU" extension, an 3integration of the "International Components for Unicode" library with 4SQLite. Documentation follows. 5 6 1. Features 7 8 1.1 SQL Scalars upper() and lower() 9 1.2 Unicode Aware LIKE Operator 10 1.3 ICU Collation Sequences 11 1.4 SQL REGEXP Operator 12 13 2. Compilation and Usage 14 15 3. Bugs, Problems and Security Issues 16 17 3.1 The "case_sensitive_like" Pragma 18 3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro 19 3.3 Collation Sequence Security Issue 20 21 221. FEATURES 23 24 1.1 SQL Scalars upper() and lower() 25 26 SQLite's built-in implementations of these two functions only 27 provide case mapping for the 26 letters used in the English 28 language. The ICU based functions provided by this extension 29 provide case mapping, where defined, for the full range of 30 unicode characters. 31 32 ICU provides two types of case mapping, "general" case mapping and 33 "language specific". Refer to ICU documentation for the differences 34 between the two. Specifically: 35 36 http://www.icu-project.org/userguide/caseMappings.html 37 http://www.icu-project.org/userguide/posix.html#case_mappings 38 39 To utilise "general" case mapping, the upper() or lower() scalar 40 functions are invoked with one argument: 41 42 upper('abc') -> 'ABC' 43 lower('ABC') -> 'abc' 44 45 To access ICU "language specific" case mapping, upper() or lower() 46 should be invoked with two arguments. The second argument is the name 47 of the locale to use. Passing an empty string ("") or SQL NULL value 48 as the second argument is the same as invoking the 1 argument version 49 of upper() or lower(): 50 51 lower('I', 'en_us') -> 'i' 52 lower('I', 'tr_tr') -> 'ı' (small dotless i) 53 54 1.2 Unicode Aware LIKE Operator 55 56 Similarly to the upper() and lower() functions, the built-in SQLite LIKE 57 operator understands case equivalence for the 26 letters of the English 58 language alphabet. The implementation of LIKE included in this 59 extension uses the ICU function u_foldCase() to provide case 60 independent comparisons for the full range of unicode characters. 61 62 The U_FOLD_CASE_DEFAULT flag is passed to u_foldCase(), meaning the 63 dotless 'I' character used in the Turkish language is considered 64 to be in the same equivalence class as the dotted 'I' character 65 used by many languages (including English). 66 67 1.3 ICU Collation Sequences 68 69 A special SQL scalar function, icu_load_collation() is provided that 70 may be used to register ICU collation sequences with SQLite. It 71 is always called with exactly two arguments, the ICU locale 72 identifying the collation sequence to ICU, and the name of the 73 SQLite collation sequence to create. For example, to create an 74 SQLite collation sequence named "turkish" using Turkish language 75 sorting rules, the SQL statement: 76 77 SELECT icu_load_collation('tr_TR', 'turkish'); 78 79 Or, for Australian English: 80 81 SELECT icu_load_collation('en_AU', 'australian'); 82 83 The identifiers "turkish" and "australian" may then be used 84 as collation sequence identifiers in SQL statements: 85 86 CREATE TABLE aust_turkish_penpals( 87 australian_penpal_name TEXT COLLATE australian, 88 turkish_penpal_name TEXT COLLATE turkish 89 ); 90 91 1.4 SQL REGEXP Operator 92 93 This extension provides an implementation of the SQL binary 94 comparision operator "REGEXP", based on the regular expression functions 95 provided by the ICU library. The syntax of the operator is as described 96 in SQLite documentation: 97 98 <string> REGEXP <re-pattern> 99 100 This extension uses the ICU defaults for regular expression matching 101 behavior. Specifically, this means that: 102 103 * Matching is case-sensitive, 104 * Regular expression comments are not allowed within patterns, and 105 * The '^' and '$' characters match the beginning and end of the 106 <string> argument, not the beginning and end of lines within 107 the <string> argument. 108 109 Even more specifically, the value passed to the "flags" parameter 110 of ICU C function uregex_open() is 0. 111 112 1132 COMPILATION AND USAGE 114 115 The easiest way to compile and use the ICU extension is to build 116 and use it as a dynamically loadable SQLite extension. To do this 117 using gcc on *nix: 118 119 gcc -fPIC -shared icu.c `pkg-config --libs --cflags icu-uc icu-io` \ 120 -o libSqliteIcu.so 121 122 You may need to add "-I" flags so that gcc can find sqlite3ext.h 123 and sqlite3.h. The resulting shared lib, libSqliteIcu.so, may be 124 loaded into sqlite in the same way as any other dynamically loadable 125 extension. 126 127 1283 BUGS, PROBLEMS AND SECURITY ISSUES 129 130 3.1 The "case_sensitive_like" Pragma 131 132 This extension does not work well with the "case_sensitive_like" 133 pragma. If this pragma is used before the ICU extension is loaded, 134 then the pragma has no effect. If the pragma is used after the ICU 135 extension is loaded, then SQLite ignores the ICU implementation and 136 always uses the built-in LIKE operator. 137 138 The ICU extension LIKE operator is always case insensitive. 139 140 3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro 141 142 Passing very long patterns to the built-in SQLite LIKE operator can 143 cause excessive CPU usage. To curb this problem, SQLite defines the 144 SQLITE_MAX_LIKE_PATTERN_LENGTH macro as the maximum length of a 145 pattern in bytes (irrespective of encoding). The default value is 146 defined in internal header file "limits.h". 147 148 The ICU extension LIKE implementation suffers from the same 149 problem and uses the same solution. However, since the ICU extension 150 code does not include the SQLite file "limits.h", modifying 151 the default value therein does not affect the ICU extension. 152 The default value of SQLITE_MAX_LIKE_PATTERN_LENGTH used by 153 the ICU extension LIKE operator is 50000, defined in source 154 file "icu.c". 155 156 3.3 Collation Sequence Security 157 158 Internally, SQLite assumes that indices stored in database files 159 are sorted according to the collation sequence indicated by the 160 SQL schema. Changing the definition of a collation sequence after 161 an index has been built is therefore equivalent to database 162 corruption. The SQLite library is well tested for robustness in 163 the fact of database corruption. Database corruption may well 164 lead to incorrect answers, but should not cause memory errors. 165