问题描述
debian@wifi:~$ echo "348dfgeccvdf" | grep -oP "\d+(?=\w{6,10})"
348
debian@wifi:~$ echo "348dfgeccvdf" | grep -oP "(?=\w{6,10})\d+"
348
对于\d+(?=\w{6,10})，这是标准的正向向前表达.
正如WiktorStribiżew在帖子中所说的那样
位置和否定后视
负向后搜索语法以(?<! and ends with the unescaped )开头.不管它出现在图案的开始，中间还是结尾，它都不会成为负面的向后看.
位置和整个前瞻(后向)表达之间可能没有关系.
For \d+(?=\w{6,10}) ,it is the standard positive look ahead expression.
As Wiktor Stribiżew say in the post
position and negative lookbehind
The negative lookbehind syntax starts with (?<! and ends with the unescaped ). Whether it appears at the start, middle or end of the pattern, it does not stop from being a negative lookbehind.
Maybe it's no realtionship between position and whole lookahead(lookbehind) expression.
根据相同的逻辑推论正面的展望，例如Wiktor Stribiewew对背后的负面展望说:
To infer for positive look ahead according to the same logic ,such as Wiktor Stribiżew say for negative lookbehind:
"\d+(?=\w{6,10})" == "(?=\w{6,10})\d+"
他们俩都对未来充满信心.
Both of them are positive look ahead.
现在这是另一个示例:
echo "adabc adabb" |grep -oP "ad(?=abc)"
ad
echo "adabc adabb" |grep -oP "(?=abc)ad"
为什么"ad(?=abc)"不等于"(?=abc)ad"?
如何解释相同结构的表达式(?=\w{6,10})\d+ and (?=abc)ad?
Why "ad(?=abc)" is not equal to "(?=abc)ad" ?
How to explain the same structure expression (?=\w{6,10})\d+ and (?=abc)ad ?
推荐答案
ad(?=abc)解释ad后跟abc，而(?=abc)ad与任何输入字符串都不匹配.导致期望'跟随abc'(?=abc)，但在正向超前后定义ad！ .模式应类似于此(?=abc)\w+，以与此输入字符串abcad
ad(?=abc) explains ad followed by abc whereas (?=abc)ad will not match with any input string. Cause expecting 'followed by abc' (?=abc) but defining ad after positive lookahead ! . Pattern should be like this (?=abc)\w+ to match with this input string abcad
(?=\w{6,10})\d+对于任何输入字符串，此模式也将与此(?=abc)ad不匹配.模式应该是这样的:
(?=\w{6,10})\d+ this pattern also will not match as this (?=abc)ad for any input string. Pattern supposed to be like this :
(?=\w{6,10})\w+和(?=abc)\w+.
这篇关于如何解释相同结构的表达式`(?= \ w {6,10})\ d +和(?= abc)ad?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！