Dialekt

Dialekt is a very small language for representing boolean expressions of the form often used by simple search engines.

Try the online demo.

Rationale and Concepts

Dialekt was designed to filter a list of entities based on sets of strings, typically human-readable “tags” or “labels”. The developers had a need describe such filters somewhat frequently, and being able to do so in a database and search-engine agnostic manner has proven useful.

Dialekt implementations are provided in a range of different programming languages. Each implementations provides:

• an expression parser, which parses Dialekt expressions into an abstract syntax tree
• a list parser, which parses space-separated tags into a set or similar data structure
• an evaluator, which checks if a given set of tags matches a specific Dialekt expression

Additionally, a mechanism allowing custom algorithms to traverse the syntax tree is provided. This can be used, for example, to generate an SQL query that finds entities with tags that match the expression.

Implementations

• PHP
• JavaScript
• Go (work in progress)
• Java (work in progress)

Syntax

The example below is a Dialekt expression that uses the entire Dialekt grammar. As you can see it is a simple language, reminiscent of SQL. You can see how this expression is parsed using the online demo.

foo bar* AND (baz OR qux) AND NOT "string \"with\" quotes"

Tags

foo, bar, baz, qux and "string \"with\" quotes" are tags. The tag is the most basic syntactic element of an expression.

Tags are a sequence of any character except whitespace, parentheses and the asterisk. Tags which include one or more asterisks form a tag pattern, which behaves similarly to SQL’s LIKE operator.

Quoted Tags

Tags may be enclosed in double-quotes, as the "string \"with\" quotes" tag has been. Doing so allows tags to include parentheses and whitespace characters.

Tags must be enclosed in double-quotes if they are equal to one of the reserved words AND, OR or NOT.

The backslash character (\) is used to escape the quote (") and asterisk (*) characters, allowing tags to include those characters directly. The backslash character itself must also be escaped in this way.

Logical Operators

An expression is composed of one or more tags, and the logical operators AND, OR and NOT. The logical operators are case-insensitive.

Two adjacent tags have an implicit AND operator. Thus, the expression foo bar is equivalent to foo AND bar.

Parentheses are used to specify the precedence of logical expressions. Where parentheses are not used, the order of precedence is NOT, AND, then OR, from highest to lowest. This, foo OR bar AND baz is equivalent to foo OR (bar AND baz).