starts
Description
The starts operator lets you filter a grammar so that only forms where the field starting with a particular string/regex are generated.
Like equals/ends/contains, rename and hide, starts applies to the content immediately to its left, but can be chained.
Usage examples
For example, we could filter A only to entries where the text starts with "p", "t", or "k":
| B = | embed | starts text |
|---|---|---|
| A | p|t|k |
Like equals, content below starts is interpreted as a regex.
For a negative example, the following filters A only to those entries where the text doesn't start with "p", "t", or "k".
| B = | embed | starts text |
|---|---|---|
| A | ~(p|t|k) |
Details
The starts operator is a convenience version of the equals operator, that converts the regex given into a more complex one. For example, in the first example above, it is converted to (p|t|k).*. In the second example, it is converted to ~((p|t|k).*).
Note the scope of the negation and the dot-star in the latter example. The starts operator does not simply tack on a dot-star to the programmer's original regex, because this would not give the meaning that the programmer probably expects. ~p.* means "Starts with the complement of p", but that's not actually what we want. The complement of p is any string that isn't p, and this includes strings like pp and the empty string. Every string starts with the empty string, so filtering with ~p.* wouldn't do anything. Instead, the appropriate regex is ~(p.*) -- "The complement of [strings starting with p]" rather than "Strings starting with [the complement of p]".
If you put a complex regex under starts, it's possible that the result of this transformation does not mean exactly what you expect it to. In that case, we suggest using the equals operator instead and writing the regex filter that you really want, rather than relying on this convenience operator.