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This article mainly introduces "how to understand modifiers in PHP regular expressions". In daily operation, I believe many people have doubts about how to understand modifiers in PHP regular expressions. I have consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful for you to answer the doubts about "how to understand modifiers in PHP regular expressions". Next, please follow the editor to study!

PHP regular expression greedy pattern:

For example, if we want to match a string that begins with the letter "a" and ends with the letter "b", but the string we need to match contains a lot of "b" after "a", such as "a bbbbbbbbbbbbbbbbb", will the regular expression match the first "b" or the last "b"? If you use greedy mode, it will match to the last "b", but only to the first "b".

Examples of PHP regular expression greedy pattern usage:

/ a.+?b/

/ a.+b/U

Examples of comparisons that do not use greed are as follows:

/ a.+b/

A modifier U is used above, as detailed in the introduction to the modifier.

What are PHP regular expressions / I, / is, / s, / isU, and so on?

I is not case sensitive

The dot metacharacter (.) in s mode matches all characters, including line breaks

The white space characters in x mode are completely ignored except those that are escaped or in the character class, and all characters, including both ends, between # and the next newline outside the unescaped character class are also ignored.

A (PCRE_ANCHORED) if this modifier is set, the pattern is forced to "anchored", which forces matching only from the beginning of the target string to automatically add ^ to the beginning of the pattern.

D (PCRE_DOLLAR_ENDONLY) if this modifier is set, the 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 this 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 will be used several times, it is worth analyzing for the sake of accelerated 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 reverses the value of the matching number so that instead of a default repetition, it follows "?" To become repetitive. This is not compatible with Perl. You can also enable this option by setting the (? U) modifier in the mode.

X (PCRE_EXTRA) this modifier enables an additional feature in PCRE that is not compatible with Perl. Any backslash in the pattern followed by a letter with no special meaning results in an error, thus retaining this combination for expansion. By default, like Perl, a backslash followed by a letter with no special meaning is treated as the letter itself. No other features are currently controlled by this modifier. That is, greedy mode, maximum matching such as: / a [\ w] +? abceade U matches abceade in abceadeddd instead of abce, if no U correction is added, then match abce u (PCRE_UTF8) this modifier enables an additional feature in PCRE that is not compatible with Perl. The pattern string is treated as UTF-8. This fix is available since PHP 4.1.0 in Unix and PHP 4.2.3 in win32.

Understanding of PHP regular expression modifiers:

The modifiers in PHP regular expressions can change many features of regular expressions, making regular expressions more suitable for your needs (note: modifiers are case-sensitive, which means that "e" is not equal to "E").

Types and introduction of PHP regular expression modifiers:

◆ I: if you add "I" to the modifier, the regular will remove case sensitivity, that is, "a" and "A" are the same.

◆ m: the default regular start "^" and end "$" is only for regular strings if you add "m" to the modifier, then the beginning and end of each line will refer to each line of the string: each line begins with "^" and ends with "$".

◆ s: if you add "s" to the modifier, the default is "." Means that any character other than a newline character will become any character, including a newline character!

◆ x: if you add this modifier, the white space character in the expression will be ignored unless it has been escaped.

◆ e: this modifier is useful only for replacement, meaning as PHP code in replacement.

◆ A: if you use this modifier, the expression must be the beginning of the matching string. For example, "/ a _ A" matches "abcd".

◆ E: contrary to "m", if you use this modifier, "$" will match the end of the absolute string instead of before the newline, which is turned on by default.

◆ U: similar to the question mark, it is used to set "greed mode".

At this point, the study on "how to understand modifiers in PHP regular expressions" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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