confused about []*

251744647

Sorry I don't have chinese input method in my laptop right now, I have to use english.

Here is the thing:  grep "[0-9]*" file  can match non digital lines such as www.co.com  
If we use: grep "[0-9]" file it only matches the lines contain numbers such as www.co1.com
As we know the regular expression [...] means to match any one of them in [].  Then how to explain [..]*           Why grep "[0-9]*" matches www.co.com?

Thanks!

Shell_HAT

* means [0-9] exists 0 time or 1 time or multi times.

251744647

Thanks!
Does [ no]* means space or n or o repeats zero or infinite times?
Like grep "can[ no]*t" file matches below lines:
I can not do it

Why? Otherwise [ no]* can mean "space plus no" exists 0 or multiple times?

回复 2# Shell_HAT


   

jason680

回复 1# 251744647

>>Why grep "[0-9]*" matches www.co.com?
   
usually the [0-9]* will matches the number, ex: 1, 23, 55,...

but the * (star) have special function with zero digit number
[0-9]* will mathes empty(nothing-- 0 digit number),or 1 digit number, 2 digits number,...

now you have to use [0-9]+ (plus) to do that

[0-9]*  ==> zero or more digits number
[0-9]+ ==> one or more digits number

Herowinter

回复 3# 251744647


   

251744647

我觉得是这样理解:[...]*中括号里面如果有两个或以上字符外面再加上*的话, 意味着匹配里面任意数目(不仅仅是任意一个)的字符,以任意顺序,匹配任意多次.举例:
grep "can[ no]t" file        didn't match  I can not do it
grep "can[ no]*t" file       matches       I can not do it
grep "can[no ]*t" file       matches       I can not do it
grep "can[on ]*t" file       matches       I can not do it
grep "can[o n]*t" file       matches       I can not do it

Shell_HAT

回复 6# 251744647


    现在还有什么问题吗？

Buring__

我去， 高大上的讨论啊

251744647

回复 7# Shell_HAT

下面的理解不知道对不对?
    [...]*中括号里面如果有两个或以上字符外面再加上*的话, 意味着匹配里面任意数目(不仅仅是任意一个)的字符,以任意顺序,匹配任意多次.

o枫叶o飘零

*表示匹配前面的子表达式零次或多次(大于等于0次)。



for  example

grep "o*"     can match  "none"  or "oo" or "ooo"  or even more "o"..



egrep "(co)*"   can match   "none"   or   "co"   or "coco" or even more "co"


egrep "[123]*"   can match "none" or "1" or "2" or "3" or "11" or "12".......



so
user alone these Expressions
[]*
()*
Character*

No significance