uni_notes/00 Inbox/29593852 - Strings.md

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created	course	topic	related	type	status	tags
2026-04-08 08:52				lecture	🔴	university

📌 Summary

[!abstract]

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📝 Content

Alphabets

Alphabets are formal, non-empty, sets of symbols (usually lowercase letters). They are denoted by Sigma.

Sigma = {a, b}

Alphabet Sigma contains the characters a and b.

Strings

A string is a set of letters. If there is an alphabet Sigma = {a,b} then abba is a string made from that alphabet. The empty string is denoted by epsilon.

Concatenation

String can be concatenated, where one string is appended to another. This operation is

Reversing / Mirroring

For a string x = a_1 a_2 ... a_(n-1) a_n of length n, it's mirrored string is given by

x^("Rev") = a_n a_(n-1)...a_2 a_1

Kleene Star

Denoted by Sigma^*. The Kleene Star (or Kleene Closure)

Formal Languages

A formal language of the alphabet Sigma is a subset L of Sigma^*