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    <p>First of all, note that there are no functions in Lua's <code>string</code> library that know anything about Unicode/mutlibyte encodings (source: Programming in Lua, 3rd edition). As far as Lua is concerned, strings are simply made up of bytes. It's up to you to figure out which bytes make up a character, if you are using UTF-8 encoded strings. Therefore, <code>string.len</code> will give you the number of <em>bytes</em>, not the number of <em>characters</em>. And <code>string.sub</code> will give you a substring of <em>bytes</em> not a substring of <em>characters</em>. </p> <p><strong>Some UTF-8 basics:</strong></p> <p>If you need some refreshing on the conceptual basics of Unicode, you should check out <a href="http://www.joelonsoftware.com/articles/Unicode.html">this article</a>.</p> <p>UTF-8 is one possible (and very important) implementation of Unicode - and probably the one you are dealing with. As opposed to UTF-32 and UTF-16 it uses a variable number of bytes (from 1 to 4) to encode each character. In particular, the ASCII characters 0 to 127 are represented with a single byte, so that ASCII strings can be correctly interpreted using UTF-8 (and vice versa, if you only use those 128 characters). All other characters start with a byte in the range from 194 to 244 (which signals that more bytes follow to encode a full character). This range is further subdivided, so that you can tell from this byte, whether 1, 2 or 3 more bytes follow. Those additional bytes are called continuation bytes and are guaranteed to be only taken from the range from 128 to 191. Therefore, by looking at a single byte we know where it stands in a character:</p> <ul> <li>If it's in <code>[0,127]</code>, it's a single-byte (ASCII) character</li> <li>If it's in <code>[128,191]</code>, it's part of a longer character and meaningless on its own</li> <li>If it's in <code>[191,244]</code>, it marks the beginning of a longer character (and tells us how long that character is)</li> </ul> <p>This information is enough to count characters, split a UTF-8 string into characters and do all sorts of other UTF-8-sensitive manipulations.</p> <p><strong>Some pattern matching basics:</strong></p> <p>For the task at hand we need a few of Lua's pattern matching constructs:</p> <p><code>[...]</code> is a character class, that matches a single character (or rather <em>byte</em>) of those inside the class. E.g. <code>[abc]</code> matches either <code>a</code>, or <code>b</code> or <code>c</code>. You can define ranges using a hyphen. Therefore <code>[\33-\127]</code> for example, matches any single one of the bytes from <code>33</code> to <code>127</code>. Note that <code>\127</code> is an escape sequence you can use in <em>any</em> Lua string (not just patterns) to specify a byte by its numerical value instead of the corresponding ASCII character. For instance, <code>"a"</code> is the same as <code>"\97"</code>.</p> <p>You can negate a character class, by starting it with <code>^</code> (so that it matches any single byte that is <em>not</em> part of the class.</p> <p><code>*</code> repeats the previous token 0 or more times (arbitrarily many times - as often as possible).</p> <p><code>$</code> is an anchor. If it's the last character of the pattern, the pattern will only match at the end of the string.</p> <p><strong>Combining all of that...</strong></p> <p>...your problem reduces to a one-liner:</p> <pre><code>local function lastChar(s) return string.match(s, "[^\128-\191][\128-\191]*$") end </code></pre> <p>This will match a character that is not a UTF-8 continuation character (i.e., that is either single-byte character, or a byte that marks the beginning of a longer character). Then it matches an arbitrary number of continuation characters (this cannot go past the current character, due to the range chosen), followed by the end of the string (<code>$</code>). Therefore, this will give you all the bytes that make up the last character in the string. It produces the desired output for all 4 of your examples.</p> <p>Equivalently, you can use <code>gsub</code> to remove that last character from your string:</p> <pre><code>function deleteLastCharacter(s) return string.gsub(s, "[^\128-\191][\128-\191]*$", "") end </code></pre> <p>The match is the same, but instead of returning the matched substring, we replace it with <code>""</code> (i.e. remove it) and return the modified string.</p>
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    1. USMartin Ender
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    1. COThanks a lot. I wish I understood what you did but will keep reading until I get it. The solution works for all cases except for Hebrew where the characters go in other direction.
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    2. COActually both work fine for hebrew. Thanks
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    3. CO@learningphp my solution is not so different from your pattern. if there is a specific thing you don't get, feel free to ask... maybe I can elaborate a bit more on that then.
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