Regular Expressions

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A regular expression describes a set of criteria that a string can match. In the validation rules of an entity a regular expression can be used to validate whether an attribute of type String matches these criteria.

A regular expression has the following properties.

Common

Name

The name can be used to refer to the regular expression from a validation rule of an entity.

Documentation

This is for documentation purpose only; it is not visible in the end-user application that you are modeling.

Expression

The expression defines the criteria that a string should be checked against in a formal, internationally standardized regular expression language.

Subexpressions

A regular expression consists of a sequence of subexpressions. A string matches a regular expression if all parts of the string match these subexpressions in the same order.

A regular expression can contain the following types of subexpressions:

  • [ ] – a bracket expression matches a single character that is indicated within the brackets. For example:

    • [abc] matches “a”, “b”, or “c

    • [a-z] specifies a range which matches any lowercase letter from “a” to “z

      These forms can be mixed: [abcx-z] matches “a”, “b”, “c”, “x”, “y”, or “z”, and is equivalent to [a-cx-z]

      The - character is treated as a literal character if it is the last or the first character within the brackets, or if it is escaped with a backslash (\)

  • [^ ] – matches a single character that is not contained within the brackets. For example:

    • [^abc] matches any character other than “a”, “b”, or “c”

    • [^a-z] matches any single character that is not a lowercase letter from “a” to “z”

      As above, literal characters and ranges can be mixed

  • {m,n} – matches the preceding element at least m and not more than n times. For example:

    • a{3,5} matches only “aaa”, “aaaa”, and “aaaaa
  • {n} – matches the preceding element exactly n times. For example:

    • [0-9]{4} ?[a-zA-Z]{2} is an alternative way to write the expression for checking the Dutch postcode in the example above
  • . – a dot matches any single character. If you want to match a dot, you can escape it by prefixing it with a \ (backslash)

  • A literal character – this is a character that does not have a special meaning in the regular expression language and it matches itself. This is effectively any character except \[](){}^-$?*+|.. For example:

    • space in the Dutch zip code example is a literal character that just matches itself

    If you need to match one of the characters which is not a literal, prefix it with a backslash (\).

  • \w – a word: a letter, digit, or underscore. \w is an abbreviation for [A-Za-z0-9_]

  • \d – a digit” an abbreviation for [0-9]

Quantifiers

The number of times that a subexpression may occur in a string is indicated by a quantifier after the subexpression. If no quantifier is present, the subexpression must occur exactly once.

The following quantifiers can be used:

Quantifier Description
? The preceding subexpression should occur not or once.
* The preceding subexpression occurs any number of times.
+ The preceding subexpression should occur once or more.
No quantifier means that the preceding subexpression should occur exactly once.

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