regex - 允许 A-Za-z0-9 的 Ruby 正则表达式

Question

我有以下正则表达式:

/([A-Za-z0-9]+)([A-Za-z0-9\-\_]+)([A-Za-z0-9]+)/

它不符合我的需要，它们是:

• 不允许有空格
• 允许大写英文字母
• 允许小写英文字母
• 允许数字
• 字符串不能同时包含连字符和下划线
• 连字符:连字符不能在字符串的开头或结尾；可以有任意数量的连字符，但连续只能有 1 个连字符(a--b 无效)。
• 下划线:下划线不能在字符串的开头或结尾；可以有任意数量的下划线，但连续只能有 1 个下划线(a__b 无效)
• 字符串必须至少包含1个字符(字母)

有效示例:

• a1_b_2_hello
• 2b-ffg-er2
• abs
• 123a

无效示例:

• _a1_b_2_hello
• 2b-ffg_er2-
• __
• --
• a__
• b--2

Answer 1

我发现将所有特殊条件放在正面和负面前瞻的开头很方便，然后在这些条件(不消耗任何字符)后面加上一般要求，此处 [a-z\d_-]+\z。

r = /
    \A           # match start of string  
    (?!.*        # begin negative lookahead and match >= 0 characters
      (?:--|__)  # match -- or __ in a non-capture group
    )            # end negative lookahead
    (?![-_])     # do not match - or _ at the beginning of the string
    (?!.*[-_]\z) # do not match - or _ at the end of the string
    (?!          # begin negative lookahead
      .*-.*_     # match - followed by _ 
      |          # or
      .*_.*-     # match _ followed by - 
    )            # end negative lookahead
    (?=.*[a-z])  # match at least one letter 
    [a-z\d_-]+   # match one or more English letters, digits, _ or -
    \z           # match end of string
    /ix          # case indifference and free-spacing modes

 "a".match? r          #=> true   
 "aB32-41".match? r    #=> true
 "".match? r           #=> false (must match a letter)
 "123-4_5".match? r    #=> false (must match a letter)
 "-aB32-4_1".match? r  #=> false (cannot begin with -)
 "aB32-4_1-".match? r  #=> false (cannot end with -)
 "_aB32-4_1".match? r  #=> false (cannot begin with _)
 "aB32-4_1_".match? r  #=> false (cannot end with _)
 "aB32--4_1".match? r  #=> false (cannot contain --)
 "aB32-4__1".match? r  #=> false (cannot contain __)
 "aB32-4_1".match? r   #=> false (cannot contain both - and _)
 "123-4_5$".match?  r  #=> false ($ is not a permitted character)

这个正则表达式按照惯例写成:

/\A(?!.*(?:--|__))(?![-_])(?!.*[-_]\z)(?!.*-.*_|.*_.*-)(?=.*[a-z])[a-z\d_-]+\z/i