I have a string which has a pattern, say "xxx.xxx".

So first three are digits, then a . and then 3 more digits.

I want to check if first digit is always 3, and next 2 can be any digits except 30 and 40, rest can be anything.

My regex attempt is:

"3[^34][0-9]."

But it doesn't seem to work. Which part here is wrong?

CodePudding user response：

Try this: ^3(?!30|40)\d{2}\.\d{3}$

Test regex here: https://regex101.com/r/iCtKnB/1

^          matches the start of string
3          matches the number 3
(?!30|40)  checks if the next two numbers are not either 30 or 40
\d{2}      matches two digits if they are not 30 or 40
\.         matches period
\d{3}      matches the next three digits
$

Basically this will match any number of your pattern which begins with 3 not followed by 30 or 40 then a dot, followed by another three digits.

In your regex: "3[^34][0-9]."

• 3 > This checks the number starts with 3
• [^34] > then the Next digit is not 3 or 4
• [0-9] > followed by one digits which can be 0 to 9
• . > followed by any other char that is not white spaces

So your requirements were not being checked in this regex.

CodePudding user response：

In case lookaheads are not supported you can use:

^3(?:[0-25-9][0-9]|[34][1-9])\.[0-9]{3}$

RegEx Demo

CodePudding user response：

You may try this:

^3(?!30|40)\d{2}\.\d{3}$

Explanation:

1. ^ asserts position at start of a line
2. 3 starts with 3
3. (?!30|40) negative look ahead to see the following two characters do not match 30 or 40 if found successful then \d{2} ensures any two digits.
4. \. means a dot
5. \d{3} means 3 digits
6. $ asserts position at the end of a line

Demo