以字母开头的正则表达式包含一个大写/一个小写字母，一个数字，没有特殊字符和最少8个字符

Question

I am trying to write a regex that:

• starts with a letter
• contains one uppercase and one lowercase letter
• contains one number
• does not allow special characters
• minimum 8 characters

So far I have the upper/lowercase conditions, the number and minimum character requirements set with the following regex:

 /^(?=.*?[a-z])(?=.*?[A-Z])(?=.*?[0-9]).{8,}$/

My best guess at resolving the starts with a letter and does not allow special characters requirements are below. This regex seems to evaluate all input to false :

/^[a-zA-Z](?=.*?[a-z])(?=.*?[A-Z])(?=.*?[0-9]).{8,}$/

Answer 1

You need to put the lookaheads after ^ and put [a-zA-Z] right after them and quantify the rest with {7,} :

^(?=.*?[a-z])(?=.*?[A-Z])(?=.*?[0-9])[a-zA-Z][a-zA-Z0-9]{7,}$

See the regex demo .

Pattern details :

• ^ - start of a string
• (?=.*?[az]) - at least 1 lowercase ASCII letter
• (?=.*?[AZ]) - at least 1 uppercase ASCII letter
• (?=.*?[0-9]) - at least 1 ASCII digit
• [a-zA-Z] - an ASCII letter
• [a-zA-Z0-9]{7,} - 7 or more ASCII letters or digits ( \\w also allows _ )
• $ - end of string.