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

I am trying to create a regex to have a string only contain 0-9 as the characters and it must be at least 1 char in length and no more than 45. so example would be 00303039 would be a match, and 039330a29 would not.

So far this is what I have but I am not sure that it is correct

[0-9]{1,45} 

I have also tried

^[0-9]{45}*$

but that does not seem to work either. I am not very familiar with regex so any help would be great. Thanks!

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.

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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.

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].

Use this regular expression if you don't want to start with zero:

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

If you don't mind starting with zero, use:

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

codaddict has provided the right answer. As for what you've tried, I'll explain why they don't make the cut:

• [0-9]{1,45} is almost there, however it matches a 1-to-45-digit string even if it occurs within another longer string containing other characters. Hence you need ^ and $ to restrict it to an exact match.

• ^[0-9]{45}*$ matches an exactly-45-digit string, repeated 0 or any number of times (*). That means the length of the string can only be 0 or a multiple of 45 (90, 135, 180...).

A combination of both attempts is probably what you need:

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

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

Rails doesnt like the using of ^ and $ for some security reasons , probably its better to use \A and \z to set the beginning and the end of the string

For this case word boundary (\b) can also be used instead of start anchor (^) and end anchor ($):

\b\d{1,45}\b

\b is a position between \w and \W (non-word char), or at the beginning or end of a string.