Example introduction to PHP regular expression modifiers

◆i: If "i" is added to the modifier, the regular expression will cancel the case sensitivity, that is, "a" and "A" are the same.
◆m: The default regular start "^" and end "$" are only for regular strings. If "m" is added to the modifier, then the start and end will refer to each line of the string: each line The beginning is "^" and the end is "$".
◆s: If "s" is added to the modifier, the default "." representing 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 end of the absolute string, not before the newline character. This mode is turned on by default.
◆U: Similar to the question mark, used to set "non-greedy mode".

Case:

"a bbbbbbbbbbbbbbbbb"
/a.+?/   匹配到a 
/a.+/U   匹配到a 
/a.+/    匹配到a bbbbbbbbbbbbbbbbb

So when U is set + the default greedy matching becomes non-greedy matching

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