# Parsing

Parsing is the process of analysing a string of characters according to the rules of a formal grammar. Parsing a SNOMED CT Expression Constraint involves processing the expression constraint string using one of the ABNF syntax specifications defined in [Chapter 5](https://github.com/SNOMED-Documents/snomed-expression-language/blob/main/7%20implementation-considerations/5.-Syntax-Specification_28739404.html), and breaking it into its constituent parts. This creates a representation of the expression constraint that can be further processed. Parsing an expression constraint is required to perform syntactic validation, concept model validation or execution. It should be noted, when parsing, that all keywords in the language are case insensitive.

A number of parser development tools are available which can generate a parser from a context-free grammar written in ABNF, such as the one defined in this document. These tools include:

* APG
* aParse
* abnfgen

Please note, the ABNF syntax defined in this specification was tested using the [APG Parser Generator](#user-content-fn-1)[^1].

Other non-ABNF parser generators are also available which can be used with an alternate syntax representation – for example:

* ANTLR
* XText
* ACE

Some of these tools (e.g. XText and ACE) can also be used to generate authoring environments with features such as syntax highlighting and autocompletion.

Alternatively, an expression constraint parser can be created manually using a programming language such as Perl or C++.

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[^1]: [www.coasttocoastresearch.com](http://www.coasttocoastresearch.com)


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