php 正则表达式加与星差？
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Regex plus vs star difference?
提问by David19801
What is the difference between:
有什么区别：
(.+?)
and
和
(.*?)
when I use it in my php preg_matchregex?
当我在我的 phppreg_match正则表达式中使用它时？
回答by stema
They are called quantifiers.
它们被称为量词。
*0 or more of the preceding expression
*0 个或多个前面的表达式
+1 or more of the preceding expression
+1 个或多个上述表达式
Per default a quantifier is greedy, that means it matches as many characters as possible.
默认情况下，量词是贪婪的，这意味着它匹配尽可能多的字符。
The ?after a quantifier changes the behaviour to make this quantifier "ungreedy", means it will match as little as possible.
在?一个量词后改变行为，使这个量词“ungreedy”，意味着它会匹配尽可能少。
Example greedy/ungreedy
贪心/不贪心的例子
For example on the string "abab"
例如在字符串“ abab”上
a.*bwill match "abab" (preg_match_all will return one match, the "abab")
a.*b将匹配“abab”（preg_match_all 将返回一个匹配项，即“abab”）
while a.*?bwill match only the starting "ab" (preg_match_all will return two matches, "ab")
whilea.*?b将只匹配起始的“ab”（preg_match_all 将返回两个匹配项，“ab”）
You can test your regexes online e.g. on Regexr, see the greedy example here
您可以在线测试您的正则表达式，例如在 Regexr 上，请参阅此处的贪婪示例
回答by Quentin
The first (+) is one or more characters. The second (*) is zero or more characters. Both are non-greedy (?) and match anything (.).
第一个 ( +) 是一个或多个字符。第二个 ( *) 是零个或多个字符。两者都是非贪婪的 ( ?) 并且匹配任何东西 ( .)。
回答by DaveRandom
A +matches one or moreinstances of the preceding pattern. A *matches zero or moreinstances of the preceding pattern.
A+匹配前面模式的一个或多个实例。A*匹配前面模式的零个或多个实例。
So basically, if you use a +there must be at least one instance of the pattern, if you use *it will still match if there are no instances of it.
所以基本上，如果你使用 a+必须至少有一个模式实例，如果你使用*它，如果没有它的实例，它仍然会匹配。
回答by Xophmeister
+matches at least one character
+匹配至少一个字符
*matches any number (including 0) of characters
*匹配任意数量（包括 0）的字符
The ?indicates a lazy expression, so it will match as few characters as possible.
该?指示慵懒的表情，所以它会匹配尽可能少的字符越好。
回答by Azri Jamil
Consider below is the string to match.
考虑下面是要匹配的字符串。
ab
The pattern (ab.*)will return a match for capture group with result of ab
该模式(ab.*)将返回捕获组的匹配结果为ab
While the pattern (ab.+)will not match and not returning anything.
虽然模式(ab.+)将不匹配并且不返回任何内容。
But if you change the string to following, it will return abafor pattern (ab.+)
但是如果将字符串更改为以下，它将返回aba模式(ab.+)
aba
回答by jeroen
+is minimal one, *can be zero as well.
+是最小的一，*也可以为零。
回答by Madara's Ghost
A star is very similar to a plus, the only difference is that while the plus matches 1 or more of the preceeding character/group, the start matches 0 or more.
星号与加号非常相似，唯一的区别是加号匹配 1 个或多个前面的字符/组，而开头匹配 0 个或多个。
回答by Miladiouss
In RegEx, {i,f}means "between ito fmatches". Let's take a look at the following examples:
在正则表达式，{i,f}意味着“之间i，以f匹配”。让我们看一下以下示例：
{3,7}means between 3 to 7 matches{,10}means up to 10 matches with no lower limit (i.e. the low limit is 0){3,}means at least 3 matches with no upper limit (i.e. the high limit is infinity){,}means no upper limit or lower limit for the number of matches (i.e. the lower limit is 0 and the upper limit is infinity){5}means exactly 4
{3,7}意味着 3 到 7 场比赛{,10}表示最多 10 场比赛，没有下限（即下限为 0）{3,}表示至少 3 个匹配，没有上限（即上限为无穷大）{,}表示匹配次数没有上限或下限（即下限为0，上限为无穷大）{5}正好是 4
Most good languages contain abbreviations, so does RegEx:
大多数优秀的语言都包含缩写，RegEx 也是如此：
+is the shorthand for{1,}*is the shorthand for{,}?is the shorthand for{,1}
+是简写{1,}*是简写{,}?是简写{,1}
This means +requires at least 1 match while *accepts any number of matches or no matches at all and ?accepts no more than 1 match or zero matches.
这意味着+需要至少 1 个匹配项，同时*接受任意数量的匹配项或根本不?接受匹配项，并且接受不超过 1 个匹配项或零个匹配项。
Credit: Codecademy.com
信用：Codecademy.com
回答by Crt
I think the previous answers fail to highlight a simple example:
我认为以前的答案未能突出一个简单的例子：
for example we have an array:
例如我们有一个数组：
numbers = [5, 15]
The following regex expression ^[0-9]+matches: 15only.
However, ^[0-9]*matches both 5 and 15. The difference is that the +operator requires at least one duplicateof the preceding regex expression
以下正则表达式^[0-9]+匹配：15仅。但是，^[0-9]*两者都匹配5 and 15。不同之处在于+运算符需要至少一个前面的正则表达式的副本
