From ef437d6584d8fd863c0dab276e5a3d4dc094767b Mon Sep 17 00:00:00 2001
From: Eduardo Julian
Date: Sun, 29 Aug 2021 21:59:48 -0400
Subject: De-taggification | part 0

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+# Appendix G: Regular expressions
+
+Working with text is a pretty common and fundamental thing in day-to-day programming.
+
+Lux's approach to doing it is with the use of composable, monadic text parsers.
+
+The idea is that a parser is a function that takes some text input, performs some calculations which consume that input, and then returns some value, and (the remaining) unconsumed input.
+
+Of course, the parser may fail, in which case the user should receive some meaningful error message to figure out what happened.
+
+The `library/lux/control/parser/text` library provides a type, and a host of combinators, for building and working with text parsers.
+
+```
+(type: .public Offset
+  Nat)
+
+(type: .public Parser
+  (//.Parser [Offset Text]))
+
+... And from library/lux/control/parser
+
+(type: .public (Parser s a)
+  (-> s (Try [s a])))
+```
+
+A good example of text parsers being used is the `library/lux/data/format/json` module, which implements full JSON serialization.
+
+---
+
+However, programmers coming from other programming languages may be familiar with a different approach to test processing that has been very popular for a number of years now: regular expressions.
+
+Regular expressions offer a short syntax to building text parsers that is great for writing quick text-processing tools.
+
+Lux also offers support for this style in its `library/lux/data/text/regex` module, which offers the `regex` macro.
+
+The `regex` macro, in turn, compiles the given syntax into a text parser, which means you can combine both approaches, for maximum flexibility.
+
+Here are some samples for regular expressions:
+
+```
+... Literals
+(regex "a")
+
+... Wildcards
+(regex ".")
+
+... Escaping
+(regex "\.")
+
+... Character classes
+(regex "\d")
+
+(regex "\p{Lower}")
+
+(regex "[abc]")
+
+(regex "[a-z]")
+
+(regex "[a-zA-Z]")
+
+(regex "[a-z&&[def]]")
+
+... Negation
+(regex "[^abc]")
+
+(regex "[^a-z]")
+
+(regex "[^a-zA-Z]")
+
+(regex "[a-z&&[^bc]]")
+
+(regex "[a-z&&[^m-p]]")
+
+... Combinations
+(regex "aa")
+
+(regex "a?")
+
+(regex "a*")
+
+(regex "a+")
+
+... Specific amounts
+(regex "a{2}")
+
+... At least
+(regex "a{1,}")
+
+... At most
+(regex "a{,1}")
+
+... Between
+(regex "a{1,2}")
+
+... Groups
+(regex "a(.)c")
+
+(regex "a(b+)c")
+
+(regex "(\d{3})-(\d{3})-(\d{4})")
+
+(regex "(\d{3})-(?:\d{3})-(\d{4})")
+
+(regex "(?<code>\d{3})-\k<code>-(\d{4})")
+
+(regex "(?<code>\d{3})-\k<code>-(\d{4})-\\0")
+
+(regex "(\d{3})-((\d{3})-(\d{4}))")
+
+... Alternation
+(regex "a|b")
+
+(regex "a(.)(.)|b(.)(.)")
+```
+
+Another awesome feature of the `regex` macro is that it will build fully type-safe code for you.
+
+This is important because the groups and alternations that you use in your regular expression will affect the type of the `regex` expression.
+
+For example:
+
+```
+... This returns a single piece of text
+(regex "a{1,}")
+
+... But this one returns a pair of texts
+... The first is the whole match: aXc
+... And the second is the thing that got matched: the X itself
+(regex "a(.)c")
+
+... That means, these are the types of these regular-expressions:
+(: (Parser Text)
+   (regex "a{1,}"))
+
+(: (Parser [Text Text])
+   (regex "a(.)c"))
+```
+
+---
+
+The benefits of parsers are that they are a bit easier to understand when reading them (due to their verbosity), and that they are very easy to combine (thanks to their monadic nature, and the combinator library).
+
+The benefits of regular expressions are their familiarity to a lot of programmers, and how quick they are to write.
+
+Ultimately, it makes the most sense to provide both mechanisms to Lux programmers, and let everyone choose whatever they find most useful.
+
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