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Regular Expressions
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.
For example, an expression for checking the Dutch postcode could be: [1-9][0-9][0-9][0-9] ?[A-Za-z][A-Za-z]
Examples: 3072AP and 7500 AH
The criteria are:
- The first character is a digit in the range 1 to 9.
- The second, third and fourth characters are digits in the range 0 to 9.
- The last two characters are letters, as expressed by the last two subexpression [A-Za-z], which indicate that the last two characters should be in the range A-Z or the range a-z.
- Between the digits and the letters there can be a space, as expressed by the subexpression which consists of a space and a question mark. The question mark indicates that the space is optional.
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:[1-9][0-9]{3} ?[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:spacein 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.\wis 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. |