Regular Expressions

Last modified: August 20, 2024

Introduction

A regular expression resource document is used in the validation rules of an entity to describe a set of criteria that a string must match.

A regular expression has the properties described below.

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 post code could be: [1-9][0-9][0-9][0-9] ?[A-Za-z][A-Za-z].

Here are two post code examples: 3072AP and 7500 AH.

These are the criteria:

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

The following sections give a summary of regular expressions that can be used in Mendix. This description also applies to regular expression strings used in functions such as isMatch().

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 post code 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:
- The space in the Dutch post 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 sub-expression should occur not or once.
*	The preceding sub-expression occurs any number of times.
+	The preceding sub-expression should occur once or more.
	No quantifier means that the preceding sub-expression should occur exactly once.

Class Pattern – information from the Oracle Java SE documentation
Using Regular Expressions in Java – information about regular expressions in Java from the Regular-Expressions.info website