# Tagged Questions

Questions about regular expressions, a formalism to describe regular languages.

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### Following yesterday's StackOverflow outage - is regular expression matching really difficult, or is the implementation simply inefficient?

Yesterday StackOverflow was down for half an hour. Later, they wrote a blog post about it, detailing that the problem stemmed from unexpectedly high complexity of regular expression matching. In ...
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### Regular languages that can't be expressed with only 2 regex operations

I thought all regular languages could be expressed with regular expressions (if a language is regular, it can be expressed with regex), but I have been told that you need all three of the regular ...
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### How do I derive a regular grammars from this regular expression? [closed]

How do I derive a regular grammars from this regular expression? (a or b)*ba(ba)* I'm stuck with the last part so ...
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### Finding a regular expression for all non-empty binary strings that contain both 0s and 1s but no consecutive 1s

This is for formal-language-style regular expression and not about Unix-style regular expression. I was trying to find the regular expression that doesn't accept empty string, doesn't accept strings ...
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### DFA for accepting all binary strings of form power of $n$ (not divisible by $n$) i.e. $n^k$ for given $n$

We can form DFA accepting binary numbers divisible by $n$. For example DFA accepting binary numbers divisible by 2 can be formed as follows: Similarly DFA accepting binary numbers divisible by 3 ...
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### Implementing regular expression matching using Brzozowski derivatives

I have been taking a language theory class, and we learned about Brzozowski derivatives recently. At class it occurred to me that they could be used to implement a simple regular expression matcher. ...
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### why DFA to regex by two different methods differ

I was learning converting DFA to regex. I came across Arden's method which solve given DFA as follows: Ardens method Let us form the equations $q_1 = q_10 + q_30 + є$ $q_2 = q_11 + q_21 + q_31$ ...