# \#lexer

**URL:** https://racket.discourse.group/tag/lexer/198.md

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## [Lexer: what means "matches no string"?](https://racket.discourse.group/t/lexer-what-means-matches-no-string/2546)

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**Author:** [@ceving](https://racket.discourse.group/u/ceving)\
**Replies:** 2\
**Last updated:** [November 28, 2023, 12:01am UTC](https://racket.discourse.group/t/lexer-what-means-matches-no-string/2546 "2023-11-28T00:01:41Z")

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nothing A lexer abbreviation that matches no string. I am not sure if this means never or always. If "no string" is an empty string, this would match always. In that case I would like to know how to stop endless rec…

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## [Lexer sub-group match](https://racket.discourse.group/t/lexer-sub-group-match/2544)

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**Author:** [@ceving](https://racket.discourse.group/u/ceving)\
**Replies:** 1\
**Last updated:** [November 27, 2023, 6:34pm UTC](https://racket.discourse.group/t/lexer-sub-group-match/2544 "2023-11-27T18:34:36Z")

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I have the following code in my lexer in order to parse hex-numbers of the format '#0f'. ((:seq #\\# (:+ (:or numeric (union-mixed "A" "B" "C" "D" "E" "F")))) (token-Hexdecimal (string-\>number (string-append (substring …

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## [Lexer matching case insensitive](https://racket.discourse.group/t/lexer-matching-case-insensitive/2543)

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**Author:** [@ceving](https://racket.discourse.group/u/ceving)\
**Replies:** 2\
**Last updated:** [November 27, 2023, 3:01pm UTC](https://racket.discourse.group/t/lexer-matching-case-insensitive/2543 "2023-11-27T15:01:22Z")

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The original SRE specification contains the definition of uncase: UNCASE is a regexp operator producing a regexp that matches any case permutation of any string that matches (: \<sre\> ...). For example, the regexp (u…