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This document uses PHP Chinese website manual Release
(PHP 4 >= 4.0.6, PHP 5)
mb_substr — 获取字符串的部分
$str
, int $start
[, int $length
= NULL
[, string $encoding
= mb_internal_encoding()
]] )
根据字符数执行一个多字节安全的 substr() 操作。
位置是从 str
的开始位置进行计数。
第一个字符的位置是 0。第二个字符的位置是 1,以此类推。
str
从该 string 中提取子字符串。
start
str
中要使用的第一个字符的位置。
length
str
中要使用的最大字符数。
If
omitted or NULL is passed, extract all characters to
the end of the string.
encoding
encoding
参数为字符编码。如果省略,则使用内部字符编码。
mb_substr() 函数根据 start
和 length
参数返回 str
中指定的部分。
[#1] qbolec at gmail dot com [2015-08-05 15:10:32]
As you often need to iterate over UTF-8 characters inside a string, you might be tempted to use mb_substr($text,$i,1).
The problem with this is that there is no "magic" way to find $i-th character inside UTF-8 string, other than reading it byte by byte from the begining. Thus a loop which calls mb_substr($text,$i,1) N times for all possible N values of $i, will take much longer than expected. The larger the $i gets, the longer is the search for $i-th letter. As characters are between 1 to 6 bytes long, one can convince oneself, that the execution time of such loop is actually Theta(N^2), which can be really slow even for moderately long texts.
One way to work around it is to first split your text into an array of letters using some smart preprocessing, and only then iterate over the array.
Here is the idea:
<?php
class Strings
{
public static function len($a){
return mb_strlen($a,'UTF-8');
}
public static function charAt($a,$i){
return self::substr($a,$i,1);
}
public static function substr($a,$x,$y=null){
if($y===NULL){
$y=self::len($a);
}
return mb_substr($a,$x,$y,'UTF-8');
}
public static function letters($a){
$len = self::len($a);
if($len==0){
return array();
}else if($len == 1){
return array($a);
}else{
return Arrays::concat(
self::letters(self::substr($a,0,$len>>1)),
self::letters(self::substr($a,$len>>1))
);
}
}
?>
As you can see, the Strings::letters($text) split the text recursively into two parts. Each level of the recursion requires time linear in the length of the string, and there is logarithmic number of levels, so the total runtime is O(N log N), which is still more than theoretically optimal O(N), but sadly this is the best idea I've got.
[#2] 247hastings at gmail dot com [2014-12-04 13:17:12]
Starting in PHP 5.4.8 passing a null as a default value to mb_substr() and mb_strcut() will work as expected.
[#3] sanjuro at 1up-games dot com [2013-09-27 18:18:22]
A serious pitfall when using mb_substr() set to HTML-ENTITIES encoding is that the function performs a number of conversions before returning the value, the worst one being that html special characters are not just counted but decoded.
<?php
mb_internal_encoding("ISO-8859-1"); echo mb_internal_encoding(),"\n<br><br>\n";
$a='jüst ä " simple " 日本 <b>test</b>';
echo mb_substr($a,0),"\n<br><br>\n";
// page source: jüst ä " simple " 日本 <b>test</b>
echo mb_substr($a,0,strlen($a),'HTML-ENTITIES');
// page source: jüst ä " simple " 日本 <b>test</b>
?>
[#4] desmatic at gmail dot com [2013-04-29 15:51:04]
quick and dirty loop through multibyte string
<?php
function get_character_classes($string, $encoding = "UTF-8") {
$current_encoding = mb_internal_encoding();
mb_internal_encoding($encoding);
$has = array();
$stringlength = mb_strlen($string, $encoding);
for ($i=0; $i < $stringlength; $i++) {
$c = mb_substr($string, $i, 1);
if (($c >= "0") && ($c <= "9")) {
$has['numeric'] = "numeric";
} else if (($c >= "a") && ($c <= "z")) {
$has['alpha'] = "alpha";
$has['alphalower'] = 'alphalower';
} else if (($c >= "A") && ($c <= "Z")) {
$has['alpha'] = "alpha";
$has['alphaupper'] = "alphaupper";
} else if (($c == "$") || ($c == "?")) {
$has['currency'] = "currency";
} else if (($c == ".") && ($has['decimal'])) {
$has['decimals'] = "decimals";
} else if ($c == ".") {
$has['decimal'] = "decimal";
} else if ($c == ",") {
$has['comma'] = "comma";
} else if ($c == "-") {
$has['dash'] = "dash";
} else if ($c == " ") {
$has['space'] = "space";
} else if ($c == "/") {
$has['slash'] = "slash";
} else if ($c == ":") {
$has['colon'] = "colon";
} else if (($c >= " ") && ($c <= "~")) {
$has['ascii'] = "ascii";
} else {
$has['binary'] = "binary";
}
}
mb_internal_encoding($current_encoding);
return $has;
}
$string = "1234asdfA?^_{}|}~????";
echo print_r(get_character_classes($string), true);
?>
Array
(
[numeric] => numeric
[alpha] => alpha
[alphalower] => alphalower
[alphaupper] => alphaupper
[currency] => currency
[ascii] => ascii
[binary] => binary
)
[#5] p dot assenov at aip-solutions dot com [2011-12-02 09:17:20]
I'm trying to capitalize only the first character of the string and tried some of the examples above but they didn't work. It seems mb_substr() cannot calculate the length of the string in multi-byte encoding (UTF-8) and it should be set explicitly. Here is the corrected version:
<?php
function mb_ucfirst($str, $enc = 'utf-8') {
return mb_strtoupper(mb_substr($str, 0, 1, $enc), $enc).mb_substr($str, 1, mb_strlen($str, $enc), $enc);
}
?>
cheers!
[#6] Anonymous [2010-02-26 05:15:46]
If start is negative, the returned string will start at the start'th character from the end of string
[#7] projektas at gmail dot com [2008-10-21 06:29:40]
First letter in upper case <hr />
<?php
header ('Content-type: text/html; charset=utf-8');
if (isset($_POST['check']) && !empty($_POST['check'])) {
echo htmlspecialchars(ucfirst_utf8($_POST['check']));
} else {
echo htmlspecialchars(ucfirst_utf8('??sin?'));
}
function ucfirst_utf8($str) {
if (mb_check_encoding($str,'UTF-8')) {
$first = mb_substr(
mb_strtoupper($str, "utf-8"),0,1,'utf-8'
);
return $first.mb_substr(
mb_strtolower($str,"utf-8"),1,mb_strlen($str),'utf-8'
);
} else {
return $str;
}
}
?>
<form method="post" action="" >
<input type="input" name="check" />
<input type="submit" />
</form>
[#8] Silvan [2007-09-01 15:30:26]
Passing null as length will not make mb_substr use it's default, instead it will interpret it as 0.
<?php
mb_substr($str,$start,null,$encoding); //Returns '' (empty string) just like substr()
?>
Instead use:
<?php
mb_substr($str,$start,mb_strlen($str),$encoding);
?>
[#9] xiaogil at yahoo dot fr [2005-08-02 08:33:36]
Thanks Darien from /freenode #php for the following example (a little bit changed).
It just prints the 6th character of $string.
You can replace the digits by the same in japanese, chinese or whatever language to make a test, it works perfect.
<?php
mb_internal_encoding("UTF-8");
$string = "0123456789";
$mystring = mb_substr($string,5,1);
echo $mystring;
?>
(I couldn't replace 0123456789 by chinese numbers for example here, because it's automatically converted into latin digits on this website, look :
零一二三四
五六七八九)
gilv
[#10] drraf at tlen dot pl [2005-02-23 06:44:00]
Note: If borders are out of string - mb_string() returns empty _string_, when function substr() returns _boolean_ false in this case.
Keep this in mind when using "===" comparisions.
Example code:
<?php
var_dump( substr( 'abc', 5, 2 ) ); // returns "false"
var_dump( mb_substr( 'abc', 5, 2 ) ); // returns ""
?>
It's especially confusing when using mbstring with function overloading turned on.