nul
Summary
The nul statement has two purposes: in the output of a rule, it signifies deleting context and keystroke, and at the start of the context
it signifies that the context buffer in the application must be empty (or no longer than the context of the rule) in order for the rule to match.
Syntax
nul [...] [+ key] > output
[...] [+ key] > nul
Description
The nul statement will delete the context and key on the left hand side of the rule from the output; it is equivalent to having an
empty output (which is not allowed). The nul statement probably will not be used often, because there are not many times you would want
to delete the context and keystroke. The nul command must be the only character or command on the right hand side of the rule.
nul can be used in the context in one special situation. It is used at the start of the context to tell Keyman to find a match only
if the nul statement is before the start of the context buffer. For example, you may use this to test if the user is typing a
character at the start of a paragraph, and use a different character in this situation.
Be careful when using nul in the context. In legacy environments, such as many Windows applications, the context buffer will be
empty after cursor movement, menu access, or many other situations. This means that the nul keyword may match in situations where
you are not expecting it, for example in the middle of a word.
In modern environments, such as touch devices, web keyboards, macOS, and some modern Windows applications, the nul keyword will
match at the start of a text buffer, and nowhere else. For example, in Microsoft Word, it will match at the start of the document, after a page,
column or section break, at the start of a header or footer, at the start of a table cell and other similar situations.
Examples
Example: Using nul on the right hand side of the context
This example will delete a visible Malayalam virama when a is typed.
U+0D4D + 'a' > nul
Example: Using nul on the left hand side of the context
This example will capitalize a letter at the start of a buffer (hint: this not really recommended!)
nul + 'a' > 'A'
Platforms
The nul statement can be used in keyboards on all platforms.
| Windows | macOS | Linux | Desktop web | Mobile web | iOS | Android |
|---|---|---|---|---|---|---|
| ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ |
Version history
The nul statement was introduced in Keyman 3.0.
