# The CommonMark\CQL class

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CommonMark Query Language is a DSL for describing how to travel through a CommonMark Node tree implemented as a parser and compiler for a small set of instructions, and a virtual machine for executing those instructions.

## Paths:

In it's most simplistic form, a CQL query combines the following paths and `/` to describe how to travel through a tree:

- firstChild
- lastChild
- previous
- next
- parent

For example, `/firstChild/lastChild` would travel to the last child node of the first child node.

## Loops

CQL can be instructed to loop, for example through the children of, or siblings to a particular node, by using the path `children`, or `siblings`. For example, `/firstChild/children` will travel to all the children of the first child node.

## Subqueries

CQL can be instructed how to travel by using a subquery like `[/firstChild]`. For example, `/firstChild/children[/firstChild]` will travel to the first child node of all the children of the first child node.

## Loop Constraints

While looping, CQL can be instructed to constrict the travelled path to nodes of particular type. For example `/children(BlockQuote)` will travel to the children of a node where the type is `BlockQuote`. The following types are recognized (case insensitively):

- BlockQuote
- List
- Item
- CodeBlock
- HtmlBlock
- CustomBlock
- Paragraph
- Heading
- ThematicBreak
- Text
- SoftBreak
- LineBreak
- Code
- HtmlInline
- CustomInline
- Emphasis
- Strong
- Link
- Image

Types may be used as a union, for example `/children(BlockQuote|List)` will travel to the children of a node where the type is `BlockQuote` or `List`. Types, or unions of types, may be also negated. For example `/children(~BlockQuote)` will travel to the children of a node where the type is not `BlockQuote`, and `/children(~BlockQuote|Paragraph)` will travel to the children of a node where the type is not `BlockQuote` or `Paragraph`

## Path Constraints

CQL can be instructed to create a loop to travel to a node of a particular type, at a particular path. For example, `/firstChild(BlockQuote)` will travel to the first child node where the type is `BlockQuote`. Note that like other loops for `children` and `siblings`, this kind of path can only be followed by a subquery.

## Implementation Notes

While CQL has been implemented as part of the PHP CommonMark extension, it stands separately from PHP and does not use PHP's virtual machine or internal representation of values.

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        CommonMark\CQL

          CommonMark\CQL

        &Constructor;

        &Methods;

&reference.cmark.commonmark.cql.construct; &reference.cmark.commonmark.cql.invoke;