What are the modifiers in PHP regular expressions?

Before learning about PHP regular expression modifiers, let’s first understand the greedy mode. As mentioned earlier in the metacharacter, "?" also has an important role, that is, "greedy mode". What is "greedy mode"? ?

PHP regular expression greedy mode:

For example, we want to match a string starting with the letter "a" and ending with the letter "b", but we need to match The string contains many "b"s after "a", such as "a bbbbbbbbbbbbbbbbb". Will the regular expression match the first "b" or the last "b"? If you use greedy mode, the last "b" will be matched, otherwise only the first "b" will be matched.

Examples of using PHP regular expression greedy mode:

/a.+?b/
/a.+b/U

Comparison examples of not using greedy mode are as follows:

/a.+b/

A modifier U is used above, please see the introduction to modifiers for details.

What are the PHP regular expressions /i, /is, /s, /isU, etc.?

i Case-insensitive

s The dot metacharacter (.) in the pattern matches all characters, including newlines

x The whitespace characters in the pattern are except Escaped or outside character classes are completely ignored, and all characters between # outside of an unescaped character class and the next newline, inclusive, are also ignored

A (PCRE_ANCHORED) If this modifier is set, the pattern is forced to be "anchored", that is, it is forced to match only from the beginning of the target string, that is, ^ is automatically added to the beginning of the pattern.

D (PCRE_DOLLAR_ENDONLY) If this modifier is set, dollar metacharacters in the pattern match only the end of the target string. Without this option, if the last character is a newline character, the dollar sign will also match before that character (but not before any other newline character). This option is ignored if the m modifier is set. There is no equivalent modifier in Perl. S When a pattern is going to be used several times, it is worth analyzing it first to speed up matching. If this modifier is set additional analysis will be performed. Currently, analyzing a pattern is only useful for non-anchored patterns that do not have a single fixed starting character.

U (PCRE_UNGREEDY) This modifier inverts the value of the number of matches so that it is not repeated by default, but becomes repeated when followed by "?". This is not compatible with Perl. This option can also be enabled by setting the (?U) modifier in the mode.

X (PCRE_EXTRA) This modifier enables an extra feature in PCRE that is incompatible with Perl. Any backslash in the pattern followed by a letter with no special meaning results in an error, thus preserving this combination for future expansion. By default, like Perl, a backslash followed by a letter with no special meaning is treated as the letter itself. No other traits are currently controlled by this modifier. That is: greedy mode, maximum matching such as: /a[\w]+?e/U matches abceade in abceadeddd instead of abce. If U is not modified, it matches abce u (PCRE_UTF8) This modifier enables a PCRE Extra functionality in Perl that is incompatible with Perl. Pattern strings are treated as UTF-8. This modifier is available since PHP 4.1.0 under Unix and since PHP 4.2.3 under win32.

Understanding of PHP regular expression modifiers:

The modifiers in PHP regular expressions can change many characteristics of regular expressions, making regular expressions more suitable for you required (note: modifiers are case-sensitive, meaning "e" is not equal to "E").

Types and introduction of PHP regular expression modifiers:

◆i: If "i" is added to the modifier, the regular expression will cancel the case sensitivity, that is, "a " is the same as "A".

◆m: The default regular start "^" and end "$" are only for regular strings. If "m" is added to the modifier, then the start and end will refer to each line of the string: The beginning of each line is "^" and the end is "$".

◆s: If "s" is added to the modifier, the default "." means that any character except the newline character will become any character, including the newline character!

◆x: If this modifier is added, whitespace characters in the expression will be ignored unless it has been escaped.

◆e: This modifier is only useful for replacement, which means it is used as PHP code in replacement.

◆A: If this modifier is used, the expression must be the beginning of the matched string. For example, "/a/A" matches "abcd".

◆E: Contrary to "m", if this modifier is used, then "$" will match the absolute end of the string, not before the newline character. This mode is turned on by default.

◆U: It has the same function as the question mark, and is used to set the "greedy mode".

The relevant content of PHP regular expression modifiers is introduced to you here. I hope it will be helpful for you to understand and master PHP regular expression modifiers.

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