为给定字符串生成 RegEx 模式

Question

I need to generate a RegEx pattern for a file name validation. But I am not used to RegEx pattern. I did try to generate patterns by using think link . I generated a pattern (^.*$)|(^.*i.*$) but it is not working as desired and got new pattern ^\\d+(?:_[A-Za-z]+)?$ from an answer but some new conditions are required which I have listed below.

My required validations are as follows :

• 1234567890 => Valid
• 1234567890_Alphabet => Valid, '_' symbol is valid, other symbol as well as white space characters are invalid.
• 1234567890 - Alphabet => Invalid
• 1234567890alphabet => Invalid

New conditions :

• 11244422_134444_john => Valid
• 1234322_1234431 john => Invalid
• 1234222_134322-john => Invalid
• 12422344_12453222 => Valid

Some other validations :

• The string should start as numeric but not alphabet. ie Numeric only
• Underscore(_) preceded by numbers.

Answer 1

Start by matching the digits, and optionally match an underscore and ASCII chars A-Za-z.

^\d+(?:_\d+)?(?:_[A-Za-z]+)?$

• ^ Start of string
• \\d+ Match 1+ digits
• (?:_\\d+)? Optionally match _ and 1+ digits
• (?:_[A-Za-z]+)? Optionally match _ and 1+ times any of A-Za-z
• $ End of string

Regex demo

Answer 2

[0-9]+($|_[A-Za-z]+)

see https://regex101.com/r/X5BMJ1/1 for live example

Basically, you get numbers first, and that's it ($) or you require _ followed by letters