不包含连续字符的正则表达式

Question

I can't figure out javascript regex that would satisfy all those requirements:

The string can only contain underscores and alphanumeric characters. It must begin with a letter, not include spaces, not end with an underscore, and not contain two consecutive underscores.

This is as far as I came, but 'not containing consecutive underscores' part is the hardest to add.

^[a-zA-Z][a-zA-Z0-9_]+[a-zA-Z0-9]$

Answer 1

You could use multiple lookaheads (neg. ones in this case):

^(?!.*__)(?!.*_$)[A-Za-z]\w*$

See a demo on regex101.com .

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Broken down this says:

 ^ # start of the line (?!.*__) # neg. lookahead, no two consecutive underscores (edit 5/31/20: removed extra Kleene star) (?!.*_$) # not an underscore right at the end [A-Za-z]\\w* # letter, followed by 0+ alphanumeric characters $ # the end

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As JavaScript snippet:

 let strings = ['somestring', '_not_this_one', 'thisone_', 'neither this one', 'but_this_one', 'this__one_not', 'this_one__yes'] var re = /^(?!.*__)(?!.*_$)[A-Za-z]\\w*$/; strings.forEach(function(string) { console.log(re.test(string)); });

Please do not restrain passwords!

Answer 2

You can also use

^[a-zA-Z]([a-zA-Z0-9]|(_(?!_)))+[a-zA-Z0-9]$

Demo

The only change comparing to your regex is changing [a-zA-Z0-9_] to [a-zA-Z0-9]|(_(?!_)) . I removed underscore from the character set and allow it in the second part of the alternative if it's not followed by another one.

(?!_) is negative lookahead meaning that _ cannot be next character

Answer 3

See regex in use here

^[a-z](?!\w*__)(?:\w*[^\W_])?$

• ^ Assert position as the start of the line
• [az] Match any lowercase ASCII letter. The code below adds the i (case-insensitive) flag, thus this also matches the uppercase variables
• (?!\\w*__) Negative lookahead ensuring two underscores do not exist in the string
• (?:\\w*[^\\W_])? Optionally match the following
  • \\w* Match any number of word characters
  • [^\\W_] Match any word character except _ . Explained: Match anything that is not not a word character, but not _ (since it's in the negated set).
• $ Assert position at the end of the line

 let a = ['somestring', '_not_this_one', 'thisone_', 'neither this one', 'but_this_one', 'this__one_not', 'this_one__yes'] var r = /^[az](?!\\w*__)(?:\\w*[^\\W_])?$/i a.forEach(function(s) { if(r.test(s)) console.log(s) });

Answer 4

甚至更简单的版本，没有环视（因此也可用于不支持它们的正则表达式风格，例如 POSIX ERE，甚至sed风格的正则表达式，只需简单更改）：

^[a-zA-Z](_?[a-zA-Z0-9]+)*$