The second link above is a great document
For a strange reason, the programmer has escaped the > (that is never needed).
Only these characters need to be escaped to obtain a literal character (outside a character class):
( ) ^ $ [ \ | . * + ?
{ # only in these cases: {n} {m,n} {m,}
# where m and n are integers
+ the pattern delimiter
Most of the time an escaped character that doesn’t need to be escaped (or that does not have a special meaning like \b \w \d …) is simply ignored by the regex engine. But it’s not the case here, because (?> is a fixed sequence to open an atomic group, and the sequence (? is not allowed except for these cases:
- a non capturing group: (?:…) a non capturing group
- an atomic group: (?>…)
An atomic group is a group that, when the regex engine exits from it, automatically throws away all backtracking positions remembered by any tokens inside the group
/a(?>(bc|b))c/
将会匹配 abcc 而不会匹配 abc
如果不是元组 而是 /a(bc|b)c/ abc 和abcc 都会匹配
元组为什么匹配不了abc呢 查看上面的 元组正则 a 匹配 a ;(?>(bc|b)) 匹配到bc 然后 abc 之后 没有任何东西匹配c 就会直接退出
而不是像非元组 那样 会在(bc|b) 创建一个岔路 一旦bc 随后的 匹配失败
还是会返回到 这个岔路上 走第二条路
再看下面例子
\b(?>integer|insert|in)\b 和 \b(?>in|integer|insert)\b
他们能不能 匹配 insert
答案是 第一个 能匹配 第二个不能匹配
- an inline modifier: (?i) (?-i) Inline modifier reference link is to ignore symbols such as capitalization
- a non capturing group with inline modifiers: (?i:…) (?-i:…)
- a lookaround: (?=…) (?!…) (?<=…) (?
# 下面是一个例子用来说明标识符的使用
$pattern = '/(?i)caseless(?-i)cased(?i)caseless/';
#等价
$pattern2 = '/(?i)caseless(?-icased)caseless/';
$str = "caselessCaseDcaselesS";
$c = preg_match_all('/(?i)caseless(?-i:Cased)caseless/', $str, $matches);
var_dump($c,$matches);The above is just to understand that only the really common or the following and some parameters are used
The above inline method can be replaced by /i /m /x
/(?i)caseless(?-i:Cased)caseless/
inline
Table 4. Common grouping syntax
Common grouping syntax
| Classification | code/syntax | illustrate |
|---|---|---|
| capture | (exp) | Match exp, and capture text into automatically named groups |
| (?exp) | Match exp, and capture the text into the group named name, can also be written as (?'name'exp) | |
| (?:exp) | Match exp, do not capture the matched text, and do not assign a group number to this group | |
| zero-width assertion | (?=exp) | matches the position before exp |
| (?<=exp) | matches the position after exp | |
| (?!exp) | matches a position that is not followed by exp | |
| (? | matches a position not preceded by exp | |
| Comment (?#comment) | This type of grouping has no effect on regular expression processing, and is used to provide comments for human reading |
# 验证正则
# ?=HT 表示 后面是HT 的位置
$str = "demoHTdmzns";
preg_match_all('/.*m.*?(?=HT)/', $str, $matches);
var_dump($matches); #会抓取到 demo
# ?<=HT 表示 前面是HT的位置
$str = "demoHTdmzns";
preg_match_all('/(?<=HT).*n/', $str, $matches);
var_dump($matches); # 会匹配到 dmzn
#?!HT 表示后面不是HT的位置
$str = "demoHTdmzns";
preg_match_all('/.*?m(?!HT)/', $str, $matches);
var_dump($matches);
# 结果如下
array(1) {
[0]=>
array(2) {
[0]=>
string(3) "dem"
[1]=>
string(5) "oHTdm"
}
}
如果是
$str = "demHTdmzns";
preg_match_all('/.*?m(?!HT)/', $str, $matches);
var_dump($matches); # 会匹配 demoHTdm
#结果如下
array(1) {
[0]=>
array(1) {
[0]=>
string(7) "demHTdm"
}
}
#?<!HT 前面不是HT 的位置
$str = "deHTdmzns";
preg_match_all('/(?<!HT)m.*/', $str, $matches);
var_dump($matches); # 会匹配 mzns
如果是
$str = "deHTdmzns";
preg_match_all('/(?<!HT)dm.*/', $str, $matches);
var_dump($matches); # 匹配为空