# What are these escape sequences in the F# compiler source?

**URL:** https://forums.fsharp.org/t/what-are-these-escape-sequences-in-the-f-compiler-source/1761
**Category:** General
**Tags:** compiler, beginners
**Created:** [August 2, 2021, 2:44pm UTC](https://forums.fsharp.org/t/what-are-these-escape-sequences-in-the-f-compiler-source/1761 "2021-08-02T14:44:06Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![leonel\_gayard](https://yyz2.discourse-cdn.com/flex030/user_avatar/forums.fsharp.org/leonel_gayard/32/868_2.png) [@leonel\_gayard](https://forums.fsharp.org/u/leonel_gayard)
#### Post date: [August 2, 2021, 2:44pm UTC](https://forums.fsharp.org/t/what-are-these-escape-sequences-in-the-f-compiler-source/1761/1 "2021-08-02T14:44:06Z")

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What are the following escape sequences in the F# compiler source? These are from file [lex.fsl](https://github.com/dotnet/fsharp/blob/v11.4.2/src/fsharp/lex.fsl#L227).

Are these escapes sequences valid F# code or are they for the lexer generator?

```auto
let letter = '\Lu' | '\Ll' | '\Lt' | '\Lm' | '\Lo' | '\Nl'

let surrogateChar = '\Cs'

let digit = '\Nd'

let connecting_char = '\Pc'

let combining_char = '\Mn' | '\Mc'

let formatting_char = '\Cf'

```

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<div class="post-metadata">

### Author: ![leonel\_gayard](https://yyz2.discourse-cdn.com/flex030/user_avatar/forums.fsharp.org/leonel_gayard/32/868_2.png) [@leonel\_gayard](https://forums.fsharp.org/u/leonel_gayard)
#### Post date: [August 2, 2021, 3:15pm UTC](https://forums.fsharp.org/t/what-are-these-escape-sequences-in-the-f-compiler-source/1761/2 "2021-08-02T15:15:20Z")

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Ah, I see. These seem to be [Unicode Character Categories](https://www.fileformat.info/info/unicode/category/index.htm)

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### Author: ![Americore](https://yyz2.discourse-cdn.com/flex030/user_avatar/forums.fsharp.org/americore/32/1031_2.png) [@Americore](https://forums.fsharp.org/u/Americore)
#### Post date: [December 13, 2021, 7:07pm UTC](https://forums.fsharp.org/t/what-are-these-escape-sequences-in-the-f-compiler-source/1761/3 "2021-12-13T19:07:20Z")

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On first sight these defined “letter” categories and sequences come across as different character related cases for the same character eg. Lu “uppercase” Ll “lowercase”.