正则表达式模式中（）和[]之间有什么区别？

Question

What is the difference between encasing part of a regular expression in () (parentheses) and doing it in [] (square brackets)?

How does this:

[a-z0-9]

differ from this:

(a-z0-9)

?

Answer 1

[] denotes a character class. () denotes a capturing group.

[a-z0-9] -- One character that is in the range of az OR 0-9

(a-z0-9) -- Explicit capture of a-z0-9 . No ranges.

a -- Can be captured by [a-z0-9] .

a-z0-9 -- Can be captured by (a-z0-9) and then can be referenced in a replacement and/or later in the expression.

Answer 2

(…) is a group that groups the contents like in math; (a-z0-9) is the grouped sequence of a-z0-9 . Groups are particularly used with quantifiers that allow the preceding expression to be repeated as a whole: a*b* matches any number of a 's followed by any number of b 's, eg a , aaab , bbbbb , etc.; in contrast to that, (ab)* matches any number of ab 's, eg ab , abababab , etc.

[…] is a character class that describes the options for one single character; [a-z0-9] describes one single character that can be of the range a – z or 0 – 9 .

Answer 3

The [] construct in a regex is essentially shorthand for an | on all of the contents. For example [abc] matches a, b or c. Additionally the - character has special meaning inside of a [] . It provides a range construct. The regex [az] will match any letter a through z.

The () construct is a grouping construct establishing a precedence order (it also has impact on accessing matched substrings but that's a bit more of an advanced topic). The regex (abc) will match the string "abc".

Answer 4

[a-z0-9] will match any lowercase letter or number. (a-z0-9) will match the exact string "a-z0-9" and allows two additional things: You can apply modifiers like * and ? and + to the whole group, and you can reference this match after the match with $1 or \\1 . Not useful with your example, though.

Answer 5

尝试（[a-z0-9]）捕获小写字母和数字的混合字符串，以及捕获后引用（或提取）。

Answer 6

[a-z0-9] will match one of abcdefghijklmnopqrstuvwxyz0123456789 . In other words, square brackets match exactly one character.

(a-z0-9) will match two characters, the first is one of abcdefghijklmnopqrstuvwxyz , the second is one of 0123456789 , just as if the parenthesis weren't there. The () will allow you to read exactly which characters were matched. Parenthesis are also useful for OR'ing two expressions with the bar | character. For example, (az|0-9) will match one character -- any of the lowercase alpha or digit.