You can use the LiveCode matchText function to check whether a string contains a specified pattern. The function returns true if the pattern matches the string, and false if not.

We need to build a regular expression that matches a valid NI number. We take the conditions described above and turn it into a regular expression

1. A UK National Insurance number begins with two prefix letters. Neither of the first two letters can be D, F, I, Q, U or V and the second letter cannot be O.

This means the first number starts with 2 letters. The first must be one of A, B, C, E, G, H, J, K, L, M, N, O, P, R, S, T, W, X, Y or Z. The second must be one A, B, C, E, G, H, J, K, L, M, N, P, R, S, T, W, X, Y or Z.

In a regex we describe this as

[A-CEGHJ-PR-TW-Z]{1}[A-CEGHJ-NPR-TW-Z]{1}

This means

exactly one letter {1} matching a characters in the list of allowed characters [A-CEGHJ-PR-TW-Z] followed by exactly one letter {1} matching a character in the list of allowed characters [A-CEGHJ-NPR-TW-Z].

2. The two prefix letters are followed by 6 digits, 0-9.

In a regex we describe this as

[0-9]{6}

3. After the 6 digits there is one suffix letter, either A, B, C or D.

In a regex we describe this as

[A-D]{1}

We put these together, in order, to get

[A-CEGHJ-PR-TW-Z]{1}[A-CEGHJ-NPR-TW-Z]{1}[0-9]{6}[A-D]{1}

Finally we need to ensure that the NI number matches the regex exactly, with no extra characters at the start or the end. This is because the matchText function checks whether a string contains the specfied pattern anywhere within it, but we want to ensure the whole string matches the pattern exactly.

We start the regex with ^, which matches the starting position within the string. Meaning the pattern matching the regex must start at the start of the string.

We end the regex with $, which matches the ending position within the string. Meaning the pattern matching the regex must end at the end of the string.

These 2 charaters together at the start and end of the regex ensure the string must match the regex exactly.

The final regular expression is

^[A-CEGHJ-PR-TW-Z]{1}[A-CEGHJ-NPR-TW-Z]{1}[0-9]{6}[A-D]{1}$

Go to the Validation tab and check the 'Validated by calculation' checkbox. Now click 'Specify' to the enter a calculation to validate the field.

Our validation step is only one line of LiveCode that returns a value so we will use LCEval to run the check.

Any newlines entered into the FileMaker `Specify Calculation` dialog are removed so when using LCEval here we need to use LiveCode's newline synonym `;` for each command. Any quotes in the script need to be escaped with FileMaker's escape character `\`

Enter the validation script into the `Specify Calculation` dialog as follows and click OK.

LCEval("
   matchText(fmParam(1), \"^[A-CEGHJ-PR-TW-Z]{1}[A-CEGHJ-NPR-TW-Z]{1}[0-9]{6}[A-D]{1}$\")
"; 
niNumber)

Entering a custom invalid entry message is a good idea to ensure users don't become frustrated if they don't know the form of a valid entry.

On the `Validation` tab of the `Field Options` dialog check the `Display custom message if validation fails` checkbox and enter a message describing the required form of a valid entry.

Test the field validation with valid and invalid entries to ensure the validation failure dialog is presented appropriately.

A valid NI number - AB123456C

An invalid NI number - QQ123456C