Questions tagged [regular-expressions]
Questions about the theory of regular expressions, both in the sense of Kleene's original definition and of POSIX regular expressions.
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Defining regular language classes with disjoint union
Regular languages are typically defined using the operations of union, concatenation, and Kleene star. Likewise, there are restricted classes of regular languages defined via similar operations, for ...
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Reference for cost of translating between regular language formalisms
It is well-known that regular languages can be defined equivalently via many formalisms, among which regular expressions, NFAs, finite monoids, Monadic Second-Order logic (MSO).
The cost (say in size ...
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Publications on left recursion and PCRE regex engine
Is there any paper (or at least technical report / preprint or even thesis) mentioning that regex engines cannot match expressions that contain "left recursion" (explained below)? I am ...
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Word equations with integer parameters
This is mainly a reference request.
Let us define a parameterized expression on a finite alphabet $\Sigma$ as follows:
$$e,e':= w\mid w^i \mid e\cdot e'$$
Where $w\in\Sigma^+$ is a word, and $i$ is an ...
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Regex: Pre-determining the position of matching characters
For all regular expressions, is it possible to pre-determine the set of possible positions in which any given sub-expression may be found? If so, is there any existing research on this subject?
Here's ...
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star height of star-free languages
I'm interested in the (restricted) star-height of star free-languages.
Recalling the definitions:
the star height $h(\mathtt{e})$ of a regular expression $\mathtt{e}$ is
$0$ if $\mathtt{e}= \...
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Regular Expressions that converts into unambiguous automata
Brüggemann-Klein and Wood (1992) proved that a certain kind of regular expressions, that they call “Deterministic Regular expressions”, when converted into automata using the Glushkov's Construction, ...
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Succinctness of regular expressions with empty word
Consider regular expressions on some alphabet $\Sigma$, without the empty word: $$e,f:=a\in\Sigma\mid e\cdot f \mid e+f\mid e^+$$
These $\varepsilon$⁻free expressions can define all regular languages ...
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Complexity of inclusion of transfinite expressions
Transfinite expressions on an alphabet $\Sigma$ are generated by the grammar :
$$e,f:= a\in\Sigma\mid e\cdot f\mid e+f\mid e^*\mid e^\omega.$$
They describe languages of transfinite words, i.e. words ...
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Ambiguity of regular expressions
Some regular expressions are ambiguous. Some are not. a*b* is unambiguous for example. Expression a*a* is ambiguous but it can ...
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Is there a list of notations developed for regular expressions? [closed]
There is, of course, PCRE.
I know also of Olin Shiver's Structural Regular Expressions, and Rob Pike's Structural Regular Expressions.
I also understand that Raku's regexps are different from perl's ...
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What graphs on $\mathbb{N}$ can be encoded as regular languages?
Suppose I represent the natural number 0 by "x", and use the symbol "s" for successor so that I get the following encoding of $\alpha : \mathbb{N} \rightarrow V$ of natural numbers ...
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Match a string agains a set of regexes
There are several algorithms to match a (simple) string against a regular expression (see here).
But if we have a lot of regexes, can we find one of them that matches the given string faster than ...
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Rewrite relations - proof of correctness
Let $T \subseteq \Sigma^* \times \Sigma^*$ be a regular relation. We define the obligatory rewrite relation over $T$ as follows:
$$
R^{obl}(T) := N(T) \cdot (T \cdot N(T))^*
$$
$$
N(T) := Id(\Sigma^* ...
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Are all RegExp solvable in O(n)?
I'm wondering if all features, that are often part of modern RegEx engines, are solvable in O(n). I'm talking about features like repeating patterns ([abc]+);\1 ...
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Kleene Algebra for star-free regular expressions
TLDR: Is there a notion of Kleene Algebra for star-free regular expressions?
Kleene Algebras are algebraic structures that are equivalent to regular expressions. A Kleene Algebra is an idempotent ...
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Time complexity of derivative-based regex matchers
Regex matching using the Brzozowski derivative without any caching or expression-simplifying takes exponential time and space because of the product rule.
In Brzozowski's original paper, Brzozowski ...
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Question About Turing Machine Computability [closed]
If p is a Turing machine then L(p) = {x | p(x) = yes}.
Let A = {p | p is a Turing machine and L(p) is a finite set}.
Is A computable? Justify your answer.
So I'm trying to figure out how to solve ...
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Shortest string in the intersection of regular languages
Inspired by https://codegolf.stackexchange.com/questions/53310/shortest-universal-maze-exit-string
Each of the 138,172 valid mazes can be represented as a DFA with 9 states (including starting and ...
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How powerful is POSIX regex
The set of languages recognized by POSIX regex is a true superset of type 3 languages. But how powerful is POSIX regex really? Is it in an already known class? Is it its own class? If so, what is the ...
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Regular expressions of prefixes/suffixes
It is well-known that star-free regular expressions, which are defined by the grammar
$r::= a \mid r \cdot r \mid r \cup r \mid \neg r \mid \varepsilon \mid \emptyset$
where $a$ belongs to a finite ...
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Searchable finite field
Let $F$ be a large finite field, where the elements are strings of length $n$. We require, addition, multiplication, and division to be efficient (polynomial in $n$).
We say that $F$ is searchable if ...
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Parameterized complexity of inclusion of regular languages
I am interested in the classic problem REGULAR LANGUAGE INCLUSION. Given a regular expression $E$, we denote by $L(E)$ the regular language associated to it. (Regular expressions are on a fixed ...
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Have people explored the problem of regular expressions being matchable? [closed]
I ran into a problem of determining if two regular expressions would have any possible matches in common. For example:
Fo*bar and Fo+bar
More specifically:
if $...
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Salomaa's axiomatisation of regular languages and the use of regular expression in it
I am reading the classical article of A. Salomaa where he gives two axiom systems for regular sets and proofs consistency and completeness.
As I have understood it, an axiomatic system in some logic (...
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Hysteresis in finite automata
The concept of hysteresis seems well suited to describe and distinguish finite automata:
"Hysteresis is the dependence of the state of a system on its history." (Wikipedia, Hysteresis)
"[The ...
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Hierarchies in regular languages
Is there any known "nice" hierarchy $L_0 \subseteq L_1 \subseteq L_2 \subseteq \dots$ (may be finite) inside the class of regular languages $L$? By nice here, the classes in each hierarchy capture ...
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For which regular expressions $\alpha$ is $\{ \beta \mid L(\alpha) = L(\beta) \}$ PSPACE-complete?
It is well known that the following problem is PSPACE-complete:
Given regular expression $\beta$, does $L(\beta) = \Sigma^*$?
What about determining equivalence to other (fixed) regular expressions $...
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Writing a regular expression for character set $\Sigma = \{a,b,(,)\}$ that not have a parenthesis inside a parenthesis [closed]
Let character set $\Sigma=\{a,b,(,)\}$. I want to write a regular expression for the language $L$ that does not have a parenthesis inside a parenthesis.
For example, $(abaab)(bbbaa) \in L$, while $(...
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Fully linear time regular expression matching
Is there an $O(n+m)$ algorithm to check whether a size $n$ regular expression matches a size $m$ string, assuming a fixed size alphabet if that matters?
The standard NFA algorithm is $O(nm)$ worst ...
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How can one ACTUALLY minimize a regular expression? [closed]
Minimizing regular expressions (in terms of number of symbols) is PSPACE-complete
(for example as discussed here: minimizing size of regular expression).
But how do you actually do that (i.e., what ...
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Complexity of DBA-recognizable Omega-Languages
Given an $\omega$-regular expression $r$, how difficult is it to decide if $L(r)$ is recognizable by some deterministic Büchi automaton? I know it is solvable in EXPTIME by converting the regular ...
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Composition of regular expressions with lookahead into DFAs
Let's say we have a regular expression ("a" | "b"(~!"b"))*, written in Perl or other similar languages that support lookahead, which should match a list of a and b's where b's are not followed by b's.
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Sub optimal regex equivalence
Regex Equivalence is a hard problem which in general takes exponential space and exponential time. Are there any approximation/sub-optimal algorithms with some theoretical guarantees over equivalence ...
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Finding containing sets, within sets of sets
Let $s = \{\sigma_1, \sigma_2 \ldots \sigma_n\}$, where $\sigma_i \in \Sigma$, denote a set of alphabet characters.
And $s \in S$ where $S$ denotes a set of sets.
Given a new set $s' = \{\sigma'_1, \...
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Is it possible to simulate a regular expression using a single stack?
I understand that a regular expression can be converted to an equivalent DFA which can then be simulated. However, is it possible to simulate the regular expression directly with the aid of a stack ? ...
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Can I show algebraically that this regular expression accepts all binary strings?
The task is to prove that (0+1)* and 0*(1.0*)* are equivalent.
1. http://rubular.com/r/K9Hp9tU6px
2. http://rubular.com/r/N8VpoEcch4
EDIT: Forgot that + was ambiguous here!
I want to prove that the ...
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Algorithm for ranking members of a regular language?
A little while back, I was reading a paper that mentioned a method for computing an integer 'rank' for a particular string $s \in L$ where $L$ is some regular language. This rank uniquely determines ...
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Chomsky hierarchy for tree structures
I know of the Chomsky hierarchy, which concerns the expressive power of grammars to recognize languages $L \subseteq \Sigma^*$ made of words on an alphabet $\Sigma$.
Is there a similar hierarchy for ...
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Deterministic Büchi + its complement covers LTL?
It is well known that deterministic Büchi automata (DBA) are less expressive than non-deterministic Büchi automata (NBA), and in particular DBA are not enough to cover linear temporal logic (LTL). ...
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minimizing size of regular expression for finite sets
It is known that minimizing the size of a regular expression is PSPACE-complete even if we have a DFA as the language's specification.
What are the results if the language is finite?
One can ...
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The Cost of an Equivalence Query for DFA
Inspired by this question, I am curious about the following:
What is the worst-case complexity of checking whether a given DFA accepts the same
language as a given regular expression?
Is this ...
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What algorithms exist for construction a DFA that recognizes the language described by a given regex?
All of my textbooks use the same algorithm for producing a DFA given a regex: First, make an NFA that recognizes the language of the regex, then, using the subset (aka "powerset") construction, ...
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Regular expressions of families of regular expressions
I was reading about the Star Height Problem and noticed that Eggan's family of regular expressions follows a simple pattern which can be described by a regular expression. My question is: are there ...
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Given a sequence find the shortest reg exp that generates it?
I'm looking for a way to find the smallest possible regular-expression that accepts a sequence.
To make it interesting I don't want any stars(Kleene stars) and preferably no wildcards?
For instance ...
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Known algorithms to go from a DFA to a regular expression
I was wondering whether there is a ``better'' (I will explain in what sense) algorithm to start from a DFA $\mathcal{A}$ and construct a regular expression $r$ such that $L(\mathcal{A})=L(r)$, than ...
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Why are regular languages called "regular"?
Why are regular languages (and from that regular expressions) called "regular"? There is lot of regularity also in context-free languages other types of languages.
I suppose that, in the beginning, ...
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Matching regular expressions using regular expressions
Is it possible to create a regular expression that matches regular expressions in any given notation?
Or, in other words, does there exist a unambiguous and full notation for regular expressions that ...
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Regular expressions without union but with backreferences
As a follow-up to my question Regular expressions without alternation, I was wondering what was known about the power of regular expressions in which union is not allowed but backreferences are.
I'm ...
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Regular expressions without alternation
I was wondering about what sets of languages are generated by restrictions of regular expressions. Supposing that all the restrictions have a constant symbol for each element of $\Sigma$ and ...
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