# Tag Info

### Unusual applications of regular expressions?

I don't know if this question belongs here (the answer could be subjective and depend on your definition of "unusual") but here is my favorite unusual application of regex: converting T9 input (2-9)...
• 593
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### Why is there no permutation in Regexes? (Even if regular languages seem to be able to do this)

The fundamental theorems of formal language theory are that regular expressions, regular grammars, deterministic finite automata (DFAs) and nondeterministic finite automata (NFAs) all describe the ...
• 80.2k
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### Proving Equivalence of Two Regular Expressions

One way to prove that two regular expressions $r_1,r_2$ generate the same language is to show both inclusions: Show that if $w$ is generated by $r_1$ then it is generated by $r_2$. Show that if $w$ ...
• 270k
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### What is the Relationship Between Programming Languages, Regular Expressions and Formal Languages

Whoever told you that regular expressions are used to parse code was spreading disinformation. Classically (I don't know to what extent this is true in modern compilers), the parsing of code – ...
• 270k
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### Difference between regular expression and grammar in automata

Regular expressions, regular grammars and finite automata are simply three different formalisms for the same thing. There are algorithms to convert from any of them to any other. The basic reason ...
• 17.7k
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### Is it possible to build any regular expression in a computer language with just 3 basic operators?

Regular expressions using only concatenation, alternation and Kleene star describe regular languages. In contrast, extended regular expressions available in modern programming languages can describe ...
• 270k
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### Why is there no permutation in Regexes? (Even if regular languages seem to be able to do this)

So my question is: (Why) Is my proof wrong? If it is right: Why is there no easy way to express permutations? Your "proof" only looked at permutations of single words, which are finite ...
Accepted

### Why are regular expressions defined with union, concatenation and star operations?

1) If we also allow intersection and complement, then the resulting expressions are sometimes called extended regular expressions; as the regular languages are closed under boolean operations nothing ...
• 1,379
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• 6,519