Regex allow a string to only contain numbers 0 - 9 and limit length to 45

Solution 1:

You are almost there, all you need is start anchor (^) and end anchor ($):

^[0-9]{1,45}$

\d is short for the character class [0-9]. You can use that as:

^\d{1,45}$

The anchors force the pattern to match entire input, not just a part of it.

Your regex [0-9]{1,45} looks for 1 to 45 digits, so string like foo1 also get matched as it contains 1.

^[0-9]{1,45} looks for 1 to 45 digits but these digits must be at the beginning of the input. It matches 123 but also 123foo

[0-9]{1,45}$ looks for 1 to 45 digits but these digits must be at the end of the input. It matches 123 but also foo123

^[0-9]{1,45}$ looks for 1 to 45 digits but these digits must be both at the start and at the end of the input, effectively it should be entire input.

Solution 2:

The first matches any number of digits within your string (allows other characters too, i.e.: "039330a29"). The second allows only 45 digits (and not less). So just take the better from both:

^\d{1,45}$

where \d is the same like [0-9].