RegEx pattern to not allow special character except underscore

Question

I have a special requirement, where i need the achieve the following

1. No Special Character is allowed except _ in between string.
2. string should not start or end with _, . and numeric value.
3. underscore should not be allowed before or after any numeric value.

I am able to achieve most of it, but my RegEx pattern is also allowing other special characters.

How can i modify the below RegEx pattern to not allow any special character apart from underscore that to in between strings.

^[^0-9._]*[a-zA-Z0-9_]*[^0-9._]$

Answer 1

Keep it simple. Only allow underscore and alphanumeric regex:

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

Javascript es6 implementation (works for React):

const re = /^[a-zA-Z0-9_]+$/;
re.test(variable_to_test);

Answer 2

What you might do is use negative lookaheads to assert your requirements:

^(?![0-9._])(?!.*[0-9._]$)(?!.*\d_)(?!.*_\d)[a-zA-Z0-9_]+$

Explanation

• ^ Assert the start of the string
• (?![0-9._]) Negative lookahead to assert that the string does not start with [0-9._]
• (?!.*[0-9._]$) Negative lookahead to assert that the string does not end with [0-9._]
• (?!.*\d_) Negative lookahead to assert that the string does not contain a digit followed by an underscore
• (?!.*_\d) Negative lookahead to assert that the string does not contain an underscore followed by a digit
• [a-zA-Z0-9_]+ Match what is specified in the character class one or more times. You can add to the character class what you would allow to match, for example also add a .
• $ Assert the end of the string

Regex demo