Reasons and solutions for failure of PHP regular matching of all characters

This article mainly shares with you the reasons and solutions for the failure of PHP regular matching of all characters. I hope it can help everyone.

<?php$str = &#39;
@@@@i ( PCRE_CASELESS)
如果设置了这个修饰符，模式中的字母会进行大小写不敏感匹配。@@@@
m ( PCRE_MULTILINE)&#39;;// 正则1：$preg = &#39;/@@@@(.*)@@@@/&#39;; 无法匹配换行符\n// 正则2：$preg = &#39;/@@@@(.*)@@@@/s&#39;;// \s   匹配任意一个空白符，等价于[\f\n\r\t\v]// \S   匹配除空白符以外任何字符，等价于[^\f\n\r\t\v]preg_match_all( $preg, $str, $matches);

print_r($matches);// 结果1：Array(
    [0] => Array()
    [1] => Array()
)// 结果2：Array(
    [0] => Array
        (
            [0] => @@@@i ( PCRE_CASELESS)
如果设置了这个修饰符，模式中的字母会进行大小写不敏感匹配。@@@@
        )
    [1] => Array
        (
            [0] => i ( PCRE_CASELESS)
如果设置了这个修饰符，模式中的字母会进行大小写不敏感匹配。
        )

)

PHP regular expression modifiers:

• i: If this modifier is set, letters in the pattern will be matched case-insensitively.

• m: By default, PCRE thinks that the target string consists of a single line of characters (however, it may actually contain multiple lines), and the "start of line" metacharacter (^) only matches characters The starting position of the string, and the "end of line" metacharacter (

  , setting this modifier has no effect.

• s: If this modifier is set, the dot metacharacter in the pattern matches all characters, including newlines. Without this modifier, the dot metacharacter does not match newlines. This modifier is equivalent to The /s modifier in perl. A negated character class such as [^a] always matches a newline character, regardless of the setting of this modifier.

• x: If set With this modifier, whitespace data characters in the pattern that are not escaped or are not in a character class are always ignored, and characters between a # character outside an unescaped character class and the next newline character are also ignored. Ignored. This modifier is equivalent to the /x modifier in Perl, allowing the compiled pattern to contain comments. Note: This only applies to data characters. Whitespace characters still cannot appear in special character sequences in the pattern, such as the sequence ( ?(Introduces a conditional subgroup (Annotation: If whitespace characters appear in the special character sequence defined by this syntax, it will cause a compilation error. For example ( ?( will cause an error.).

• e: If this modifier is set, preg_replace(), after performing the back reference replacement of the replacement string, evaluates and executes the replaced string as PHP code (eval function method), and uses the execution result as The string that actually participates in the replacement. Single quotes, double quotes, backslash () and NULL characters will be escaped with backslashes when backreference replacement.

  Tip
  请确保replacement参数由合法php代码字符串组成, 否则php将会 在preg_replace()调用的行上 产生一个解释错误.
  Note: 仅 preg_replace()使用此修饰符, 其他PCRE函数忽略此修饰符.

• A : If this modifier is set, the pattern is forced to be an "anchored" pattern, which means that the match is constrained to search only from the beginning of the target string. This effect can also be constructed using the appropriate pattern, and is also perl The only way to implement this pattern.

• D: If this modifier is set, the metacharacter dollar sign in the pattern only matches the end of the target string. If this modifier is not Set, when the string ends with a newline character, the dollar sign will also match that newline character (but will not match any previous newline character). If the modifier m is set, this modifier is ignored. In perl, there is no and This modifier is equivalent to the modifier .

• #S: When a pattern needs to be used multiple times, in order to improve the matching speed, it is worth spending some time to perform some additional analysis on it. . If this modifier is set, this additional analysis will be performed. Currently, this analysis of a pattern only applies to non-anchored pattern matches (i.e. without a single fixed start character).

• U: This modifier reverses the "greedy" mode of the quantifier. It makes the quantifier non-greedy by default. It can be made greedy by following the quantifier?. This is incompatible with perl. . It can also be set using an in-mode modifier setting (?U), or a question mark after the quantifier to mark it as non-greedy (e.g. .*?).

  Note:
  
  在非贪婪模式, 通常不能匹配超过 pcre.backtrack_limit的字符.

• X: This The modifier turns on the attachment functionality of PCRE which is incompatible with Perl. Any backslash in the pattern followed by a character with no special meaning will result in an error, these characters are preserved to ensure backward compatibility. By default, In Perl, a backslash followed by a character with no special meaning is considered the original text of that character. Currently no other features are controlled by this modifier.

• J: Internal option setting (?J) Modify the local PCRE_DUPNAMES option. Allow subgroups with duplicate names. (Annotation: Can only be set through internal options, external /J settings will cause errors.)

• u: This modifier turns on an additional feature that is incompatible with Perl. Pattern strings are considered to be UTF-8. This modifier is available starting with the Unix version of PHP 4.1.0 or higher, and the Win32 version of PHP 4.2.3. PHP 4.3. 5 Start checking the utf-8 legality of the schema.

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