String validation using regular expressions

Question

I need to create a regex for 3 rules.

The string can start with a letter or _. No numbers are allowed before the letter or _.
The _ can be added in any place.
Using special characters or spaces is not allowed.

I created 2 regexes(^[A-Za-z0-9_]*$ and ^[^0-9]*$) but don't know how to combine them together. Also, the rules do not include a number at the end(like _Sss12).

_Aaa12 - correct
Aaa    - correct
aa aa  - fail
12aa   - fail

Sounds like you are verifying identifiers. Try ^[A-Za-z_][A-Za-z0-9_]*$. — InSync
– InSync, Commented Oct 2, 2023 at 22:08
@InSync Comments aren't for answers. Comments are for clarifying questions. If you have an answer, post it below in an answer. — JeffC
– JeffC, Commented Oct 2, 2023 at 22:10
@JeffC I know that. I'm finding a good target to VTC as dupe. — InSync
– InSync, Commented Oct 2, 2023 at 22:12
Possible duplicates: Regular expression to recognize variable declarations in C (the regex in the question itself), Java Regular expression for checking validity of Variable name, Regular expression for variables identificators. — InSync
– InSync, Commented Oct 2, 2023 at 22:14

Dmitrii Bychenko · Accepted Answer · 2023-10-02 22:10:47Z

3

As far as I can see, you want to match an identifier, which

must start from letter A..Z, a..z or _
can contain letters A..Z, a..z or _, or digits 0..9

If it's your case then the pattern can be

^[A-Za-z_][A-Za-z0-9_]*$

where

^             - anchor, start of the string
[A-Za-z_]     - Letter A..Z or a..z or symbol _
[A-Za-z0-9_]* - Zero or more letters, digits or _
$             - anchor, end of the string

If letters can be any Unicode letter, not necessary Latin ones, you can use \p{L} (for any Unicode digit, not necessary 0..9 \d can be used, but I doubt if you want Persian or Indian digits in the pattern):

^[\p{L}_][\p{L}0-9_]*$

edited Oct 2, 2023 at 22:10

answered Oct 2, 2023 at 22:05

Dmitrii Bychenko

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