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模式修正符

模式修正符

模式修正符 — 解说正则表达式模式中使用的修正符

说明

下面列出了当前在 PCRE 中可能使用的修正符。括号中是这些修正符的内部 PCRE 名。修正符中的空格和换行被忽略,其它字符会导致错误。

i (PCRE_CASELESS)
如果设定此修正符,模式中的字符将同时匹配大小写字母。
m(PCRE_MULTILINE)
默认情况下,PCRE 将目标字符串作为单一的一“行”字符所组成的(甚至其中包含有换行符也是如此)。“行起始”元字符(^)仅仅匹配字符串的起始,“行结束”元字符($)仅仅匹配字符串的结束,或者最后一个字符是换行符时其前面(除非设定了 D 修正符)。这和 Perl 是一样的。

当设定了此修正符,“行起始”和“行结束”除了匹配整个字符串开头和结束外,还分别匹配其中的换行符的之后和之前。这和 Perl 的 /m 修正符是等效的。如果目标字符串中没有“\n”字符或者模式中没有 ^ 或 $,则设定此修正符没有任何效果。

s(PCRE_DOTALL)
如果设定了此修正符,模式中的圆点元字符(.)匹配所有的字符,包括换行符。没有此设定的话,则不包括换行符。这和 Perl 的 /s 修正符是等效的。排除字符类例如 [^a] 总是匹配换行符的,无论是否设定了此修正符。
x(PCRE_EXTENDED)
如果设定了此修正符,模式中的空白字符除了被转义的或在字符类中的以外完全被忽略,在未转义的字符类之外的 # 以及下一个换行符之间的所有字符,包括两头,也都被忽略。这和 Perl 的 /x 修正符是等效的,使得可以在复杂的模式中加入注释。然而注意,这仅适用于数据字符。空白字符可能永远不会出现于模式中的特殊字符序列,例如引入条件子模式的序列 (?( 中间。
e
如果设定了此修正符,preg_replace() 在替换字符串中对逆向引用作正常的替换,将其作为 PHP 代码求值,并用其结果来替换所搜索的字符串。

只有 preg_replace() 使用此修正符,其它 PCRE 函数将忽略之。

注: 本修正符在 PHP3 中不可用。

A(PCRE_ANCHORED)
如果设定了此修正符,模式被强制为“anchored”,即强制仅从目标字符串的开头开始匹配。此效果也可以通过适当的模式本身来实现(在 Perl 中实现的唯一方法)。
D(PCRE_DOLLAR_ENDONLY)
如果设定了此修正符,模式中的美元元字符仅匹配目标字符串的结尾。没有此选项时,如果最后一个字符是换行符的话,美元符号也会匹配此字符之前(但不会匹配任何其它换行符之前)。如果设定了 m 修正符则忽略此选项。Perl 中没有与其等价的修正符。
S
当一个模式将被使用若干次时,为加速匹配起见值得先对其进行分析。如果设定了此修正符则会进行额外的分析。目前,分析一个模式仅对没有单一固定起始字符的 non-anchored 模式有用。
U(PCRE_UNGREEDY)
本修正符反转了匹配数量的值使其不是默认的重复,而变成在后面跟上“?”才变得重复。这和 Perl 不兼容。也可以通过在模式之中设定 (?U) 修正符或者在数量符之后跟一个问号(如.*?)来启用此选项。
X(PCRE_EXTRA)
此修正符启用了一个 PCRE 中与 Perl 不兼容的额外功能。模式中的任何反斜线后面跟上一个没有特殊意义的字母导致一个错误,从而保留此组合以备将来扩充。默认情况下,和 Perl 一样,一个反斜线后面跟一个没有特殊意义的字母被当成该字母本身。当前没有其它特性受此修正符控制。
u(PCRE_UTF8)
此修正符启用了一个 PCRE 中与 Perl 不兼容的额外功能。模式字符串被当成 UTF-8。本修正符在 Unix 下自 PHP 4.1.0 起可用,在 win32 下自 PHP 4.2.3 起可用。自 PHP 4.3.5 起开始检查模式的 UTF-8 合法性。
add a note add a noteUser Contributed Notes

varrah NO_GARBAGE_OR_SPAM AT mail DOT ru
03-Nov-2005 08:12

Spent a few days, trying to understand how to create a pattern for Unicode chars, using the hex codes. Finally made it, after reading several manuals, that weren't giving any practical PHP-valid examples. So here's one of them:

For example we would like to search for Japanese-standard circled numbers 1-9 (Unicode codes are 0x2460-0x2468) in order to make it through the hex-codes the following call should be used:
preg_match('/[\x{2460}-\x{2468}]/u', $str);

Here $str is a haystack string
\x{hex} - is an UTF-8 hex char-code
and /u is used for identifying the class as a class of Unicode chars.

Hope, it'll be useful.

hfuecks at nospam dot org
15-Jul-2005 10:14

Regarding the validity of a UTF-8 string when using the /u pattern modifier, some things to be aware of;

1. If the pattern itself contains an invalid UTF-8 character, you get an error (as mentioned in the docs above - "UTF-8 validity of the pattern is checked since PHP 4.3.5"

2. When the subject string contains invalid UTF-8 sequences / codepoints, it basically result in a "quiet death" for the preg_* functions, where nothing is matched but without indication that the string is invalid UTF-8

3. PCRE regards five and six octet UTF-8 character sequences as valid (both in patterns and the subject string) but these are not supported in Unicode ( see section 5.9 "Character Encoding" of the "Secure Programming for Linux and Unix HOWTO" - can be found at http://www.tldp.org/ and other places )

4. For an example algorithm in PHP which tests the validity of a UTF-8 string (and discards five / six octet sequences) head to: http://hsivonen.iki.fi/php-utf8/

The following script should give you an idea of what works and what doesn't;

<?php
$examples
= array(
'Valid ASCII' => "a",
'Valid 2 Octet Sequence' => "\xc3\xb1",
'Invalid 2 Octet Sequence' => "\xc3\x28",
'Invalid Sequence Identifier' => "\xa0\xa1",
'Valid 3 Octet Sequence' => "\xe2\x82\xa1",
'Invalid 3 Octet Sequence (in 2nd Octet)' => "\xe2\x28\xa1",
'Invalid 3 Octet Sequence (in 3rd Octet)' => "\xe2\x82\x28",

'Valid 4 Octet Sequence' => "\xf0\x90\x8c\xbc",
'Invalid 4 Octet Sequence (in 2nd Octet)' => "\xf0\x28\x8c\xbc",
'Invalid 4 Octet Sequence (in 3rd Octet)' => "\xf0\x90\x28\xbc",
'Invalid 4 Octet Sequence (in 4th Octet)' => "\xf0\x28\x8c\x28",
'Valid 5 Octet Sequence (but not Unicode!)' => "\xf8\xa1\xa1\xa1\xa1",
'Valid 6 Octet Sequence (but not Unicode!)' => "\xfc\xa1\xa1\xa1\xa1\xa1",
);

echo "++Invalid UTF-8 in pattern\n";
foreach (
$examples as $name => $str ) {
echo
"$name\n";
preg_match("/".$str."/u",'Testing');
}

echo "++ preg_match() examples\n";
foreach (
$examples as $name => $str ) {

preg_match("/\xf8\xa1\xa1\xa1\xa1/u", $str, $ar);
echo
"$name: ";

if ( count($ar) == 0 ) {
echo
"Matched nothing!\n";
} else {
echo
"Matched {$ar[0]}\n";
}

}

echo "++ preg_match_all() examples\n";
foreach (
$examples as $name => $str ) {
preg_match_all('/./u', $str, $ar);
echo
"$name: ";

$num_utf8_chars = count($ar[0]);
if (
$num_utf8_chars == 0 ) {
echo
"Matched nothing!\n";
} else {
echo
"Matched $num_utf8_chars character\n";
}

}
?>

csaba at alum dot mit dot edu
09-Apr-2005 08:40

Extracting lines of text:

You might want to grab a line of text within a multiline piece of text.  For example, suppose you want to replace the first and last lines within the <body> portion of a web $page with your own $lineFirst and $lineLast.  Here's one possible way:

<?php
$lineFirst
= "This is a new first line<br>\r\n";
$lineLast  = "This is a new last line<br>\r\n";
$page = <<<EOD
<html><head>
<title>This is a test page</title>
</head><body>
This is the first line<br>
Hi Fred<br>
Hi Bill<br>
This is the last line<br>
</body>
</html>
EOD;
$re = "/<body>.*^(.+)(^.*?^)(.+)(^<\\/body>.*?)/smU";
if (
preg_match($re, $page, $aMatch, PREG_OFFSET_CAPTURE))
$newPage = substr($text, 0, $aMatch[1][1]) .
$lineFirst . $aMatch[2][0] .
$lineLast . $aMatch[4][0];
print
$newPage;
?>

The two (.+) are supposed to match the first and last lines within the <body> tag.  The /s option (dot all) is needed so the .* can also match newlines.  The /m option (multiline) is needed so that the ^ can match newlines.  The /U option (ungreedy) is needed so that the .* and .+ will only gobble up the minimum number of characters necessary to get to the character following the * or +.  The exception to this, however, is that the .*? temporarily overrides the /U setting on .* turning it from non greedy to greedy.  In the middle, this ensures that all the lines except the first and last (within the <body> tag) are put into $aMatch[2].  At the end, it ensures that all the remaining characters in the string are gobbled up, which could also have been achieved by .*)\\z/ instead of .*?)/

Csaba Gabor from Vienna

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