The regular expression that matches strings of certain length that consist of at least one non-whitespace character
by yaobin.wen
The system I’m working on has an input field that we use exactly one regular expression to test if the given value is valid or not. The validity tests are:
- 1). It can’t be an empty string, i.e., the length must be greater than zero.
- 2). The string consists of at most 10 characters.
- 3). At least one character must be a non-whitespace character.
For example, the following values are acceptable (note that \w refers to a whitespace which is either a blank space or a tab):
| Value | Acceptable? | Notes |
|---|---|---|
"0123" |
Yes | Less than 10 characters. |
"0123456789" |
Yes | Exactly 10 characters. |
"0123456789abc" |
No | Longer than 10 characters. |
"\w\w\w" |
No | Only white spaces |
"A\w\w\w\w\w\w" |
Yes | A lot of white spaces but has at least an non-whitespace character A. |
"\w\w\w\wA" |
Yes | A lot of white spaces but has at least an non-whitespace character A. |
"\w\w\wA\w\w\w" |
Yes | A lot of white spaces but has at least an non-whitespace character A. |
The first regular expression that bumped into my head was ^.*\S.*$ which means:
\S: There must be one non-whitespace character..*before\S: There can be an arbitrary number of any characters before the non-whitespace character..*after\S: There can be an arbitrary number of any characters after the non-whitespace character.
This regular expression can satisfy the validity tests 1) and 3). Unfortunately, it doesn’t limit the string length to 10 at most. As a result, the string 01234567890123456789 will also pass the validity test.
This answer that uses negative lookahead inspired and helped me figure out the solution to my problem: ^(?!\s*$).{1,10}$. You can try this regular expression to any one of the following online regular expression testing websites:
Using negative lookbehind ^(?<!\s*$).{1,10}$ may not work because the * quantifier inside a lookbehind makes it non-fixed width, but, as mentioned in Lookahead and Lookbehind Zero-Length Assertions, many regular expression implementations “including those used by Perl, Python, and Boost only allow fixed-length strings.”