Regex for IBAN allowing for white spaces AND checking for exact length

Question

I need to check an input field for a German IBAN. The user should be allowed to leave in white spaces and input should be validated to have a starting DE

Answer 1

Because you should use the right tool for the right task: you should not rely on regexps to validate IBAN numbers, but instead use the IBAN checksum algorithm to check the whole code is actually correct, making any regexp superfluous and redundant. i.e.: remove all spaces, rearrange the code, convert to integers, and compute remainder, here it's best explained.

Though, there am I trying to answer your question, for the fun of it:

what about:

^DE([0-9a-zA-Z]\s?){20}$

which only difference is allowing a whitespace (or not) after each occurence of a alphanumeric character.

here is the visualization:

edit: for the OP's information, the only difference is that this regexp, from @ulugbex-umirov: (?:\s*[0-9a-zA-Z]\s*) does a lookahead check to see if there's a space between the iso country code and the checksum (which only made of numerical digits), which I do not support on purpose.

And actually to support a correct IBAN syntax, which is formed of groups of 4 characters, as the wikipedia page says:

^DE\d{2}\s?([0-9a-zA-Z]{4}\s?){4}[0-9a-zA-Z]{2}$

example

If your UI is in Javascript, you can use that library for doing IBAN validation:

so you know this is a valid IBAN, and can validate it before the data is ever even sent to the backend. Simpler, lighter and more elegant… Why do something else?