Text that includes ONLY letters, digits, and the following characters: _-.

仅包括字母，数字和这些字符： _-。

youdao

Field names can contain only letters, Numbers, or the underscore character.

字段名称只能包含字母、数字或下划线字符。

youdao

Post-processing the text of only letters and numbers for higher-order data is a much easier method of developing a useful annotation system with available open source systems.

要使用开源系统开发实用的注释系统，对只有字母和数字组成的文本进行后处理以获得更有条理的数据，这种方法更加简便。

youdao

The only evidence was cut into a tree and a fence: the letters C-R-O and the word Croatoan,C-R-O-A-T-O-A-N.

VOA: special.2009.08.17

But he seemed to only include ten letters in his list.

他似乎只把十封信列入了他的清单里。

耶鲁公开课 - 新约课程节选

As far as how many letters, nobody can tell for sure whether the letters sent and recovered were the only letters.

CNN: "The Secret Life of Germs" author Philip Tierno

Only letters that are used more than 80% of the time are shown, and the rest of the keys are hidden to keep the user experience crisp, Mr. Ghassabian said. (The Z key is the least-frequently used, followed closely by the X key.) He says the system is up to three times faster than a traditional keyboard.

WSJ: The Future of Typing

It said 17, 000 people had signed a petition expressing their support for the plan and the council had received 1, 100 supporting letters and "only 230 letters in opposition".

BBC: Leigh-on-Sea Havens Hospice plan turned down