REGEX in Salesforce validation rules - BIC verification

The following REGEX works in https://regex101.com/ but not in Salesforce.

^[a-z]{6}[0-9a-z]{2}([0-9a-z]{3})?\z

Source: https://stackoverflow.com/questions/15920008/regex-for-bic-check

It checks whether a given BIC :

• has 8 or 11 characters, of which
• first 6 must be alphabetic (4 bank code + 2 country code)
• next 2 must be alphanumeric (location code)
• last 3 are optional, alphanumeric (branch code)

Examples

Correct: AGRIFRPP882

Correct: NOLADE21STS

Correct: OPSKATWW

Incorrect: OPSKATW

Incorrect: AABB12CC

When using REGEX in Salesforce, backslash characters \ must be changed to double backslashes \\ because backslash is an escape character in Salesforce. The expression must also be put within quotation marks. See Salesforce help.

Thus the validation rule would be: 

NOT(REGEX(BIC__c, "^[a-z]{6}[0-9a-z]{2}([0-9a-z]{3})?\\z"))

However, when checking on AGRIFRPP882, which is a correct BIC, the validation rule always fires.

  #REGEX #Validation Rule