We are in regular communication by letter.

我们定期通信联系。

《牛津词典》

Both single-quoted or triple-quoted strings may be preceded by the letter "r" to indicate that regular expression special characters should not be interpreted by Python. For example.

使用单引号或三重引号的字符串前面可以加一个字母“ r ”以表示Python不应该解释规则表达式特殊字符。

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And you can use letter collections (as you might in a regular expression), for example, to list the files with a "c" or "o" extension, as shown in Listing 2.

并且您可以使用字母集合(就像您可能在正则表达式中使用的那样)，例如，用于列出具有“c”或“o”扩展名的文件，如清单2所示。

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The cost of a second-class stamp for a regular letter increased from 36p to 50p.

BBC: Price cap for parcels confirmed by mail regulator Ofcom

Its paper tray handles 150 sheets of regular letter size paper.

FORBES: Small and Affordable Brother Multifunction Device Prints 11 x 17-inch pages

Some 49 groups - including the Federation of Scottish Theatre, the Scottish National Jazz Orchestra and the Edinburgh Mela - were told by letter that their regular funding would end next spring.

BBC: Scotland arts correspondent