# Tagged Questions

181 views

### Can regexp be used to check for palindromes?

Can regexp pattern matching be used to check for palindromes within a given text in Java? My intuition is that in order to check for palindromes, we may need to remember the character that was ...
86 views

### Writing a simple code validator

I know that programming languages can be defined in EBNF which can be converted into regular expressions. Right now I am working on a very simple BASIC interpreter for a project. The code has to be ...
129 views

### How to create an algorithm for determining Boolean Tautologies

So i am right now exploring some topics in a proof course and it occurred to me to try to create a boolean tautology solver. I would like an algorithm that is more efficient than brute force. Problem ...
39 views

### Generating all the words accepted by a deterministic finite automaton [duplicate]

Imagine you've got a finite deterministic automaton A defining some finite regular language L, consisting of N words. The first question is: how would you determine the number of words N? Then, I ...
100 views

### Need overview of concepts and tools to translate a DSL to regular expressions

I'm looking for a little guidance. Until this morning, this was all over my head. After spending today researching Wikipedia, StackOverflow, etc., I'd say I've got my nose above the water. I'm tasked ...
671 views

### Regex to String generation

Let's say that we have a regex and an index i. If we suppose that the set of strings that match our regex are sorted in a lexicographical order, how could we get the i element of this list? Edit : I ...
89 views

### Why does the regular expression metacharacter \w include underscores?

In most, if not all implementations of regular expressions, the \w metacharacter matches all alphanumeric characters plus the underscore. Historically speaking, why was the underscore character ...
203 views

### Implementing a basic interpreter - should I learn regular expressions? [closed]

For fun and for practice, I'm thinking of maybe implementing a very basic programming language to run on a simple interpreter written in Java. My question is this: Is it a 'must' to know regular ...
318 views

### Word recognition in a string without spaces or punctuation marks

I have a small C# project that reads a file and gives me an output: a string that does not contain spaces nor any types of punctuation marks. It may also contain a few misspellings. Ex. Output: ...
140 views

### What is the right way to parse HTML? [closed]

I've heard, that parsing HTML using the Cthulhu way is not very good. But what are the right ways to parse HTML? Or is it possible to parse it at all?
1k views

### Should I refactor large functions that mostly consist of one regex? [closed]

I just wrote a function that spans approximately 100 lines. Hearing that, you are probably tempted to tell me about single responsibilities and urge me to refactor. This is my gut instinct as well, ...
169 views

### Regular Expressions and Language Generators

I have few CS textbooks with me which discuss languages, well actually 2 plus old course notes supplied a few years ago. I have been searching the web too any only seem to come up with vague responses ...
60 views

### Nomenclature for wildcard vs regex expressions [closed]

When I see the term pattern in a description of syntax I infer a regular expression is being specified. Is there a separate term that specifies a wildcard expression? ^foo.*(bar|baz){3} <-- ...
17k views

### What is meant by “Now you have two problems”?

There is a popular quote by Jamie Zawinski: Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems. How is this quote supposed to ...
324 views

### Are there any implementations of deterministic regular expressions? [closed]

The question itself is already in the title, so here I will just provide additional details. I call a regular expression "deterministic", if, after converting the regular expression into a ...
236 views

### Multi regex matching — what to do to optimize it?

I have multiple regexes matching an input in greedy-first mode. All regexes are already compiled and I don't see what I can do more to speed-up things. I mean using tools coming with .Net framework. ...
220 views

### Object oriented parser model review

First time I make use of Code Review. At the moment I'm creating an Android application and the company I'm developing it for, has a specific file format that can be used to give text based data ...
135 views

### Building a regex builder [closed]

I am a beginner in programming in general and web development in particular. I am especially bad at regular expressions. Recently I was involved in building a couple of cPanel plugins(Perl-CGI) and ...
977 views

### Java regex patterns - compile time constants or instance members?

Currently, I have a couple of singleton objects where I'm doing matching on regular expressions, and my Patterns are defined like so: class Foobar { private final Pattern firstPattern = ...
299 views

### Optimized Special Character Escaper vs Matcher/Pattern

I need to escape special characters which are sent to apache lucent. Since the code will run on a production server I want the code to be the fastest possible. I've seen multiple ways to do it: ...
105 views

### How does the ? make a quantifier lazy in regex [closed]

I've been looking into regex lately and figured that the ? operator makes the *,+, or ? lazy. My question is how does it do that? Is it that *? for example is a special operator, or does the ? have ...
275 views

### pcre case insensitivity: handle escape sequences as raw bytes?

Consider the following regex (no unicode): Example:\x04\x05\x41 Suppose you search this regex case-insensitively. Would you expect it to search the final \x41 in case-sensitive manner? People whom ...
704 views

### Why nginx's http parser doesnt use regular expressions?

I see the http parser written by Igor Sysoev for nginx does not use regular expressions https://github.com/joyent/http-parser What could be the main reason for such design decision? I guess I could ...
4k views

### Readable regular expressions without losing their power?

Many programmers know the joy of whipping up a quick regular expression, these days often with help of some web service, or more traditionally at interactive prompt, or perhaps writing a small script ...
372 views

### Domain-specific language for text search/processing?

I work for an organization that does a lot of work with government data. We have a couple of different projects where we've abstracted out common text search/manipulation operations into reusable ...
372 views

### How to choose a proper parser generator for PHP

Some programmers avoid regexes in some situations (see this popular @nickf comment), perhaps using a parsing framework such as Lex/Yacc. Others prefer to stay within PHP, perhaps using regular ...
322 views

### Constructing a Finite State Automaton

I have an exam question that I am unsure of the answer. The question is: In organisation X valid user names have the following structure. The user name can be either the employee’s name followed ...
683 views

### Constructing a Deterministic Finite State Automaton for a given Regex

I have a couple of exam questions for my compilers class and wanted to check if my solutions are correct. The first question is: Consider a language in which numbers start with an optional minus ...
198 views

### Can non-IT people learn and take advantage of regular expressions? [closed]

Often times, not-IT people has to deal with massive text data, clean it, filter it, modify it. Often times normal office tools like Excel lack the tools to make complex search and replace operations ...
4k views

### Can the csv format be defined by a regex?

A colleague and I have recently argued over whether a pure regex is capable of fully encapsulating the csv format, such that it is capable of parsing all files with any given escape char, quote char, ...
82 views

### String patterns that can be used to filter and group files

One of our application filters files in certain directory, extract some data from it and export a document from the extracted data. The algorithm for extracting the data depends on the file, and so ...
2k views

### Are regular expressions a programming language?

In the academic sense, do regular expressions qualify as a programming language? The motivation for my curiosity is an SO question I just looked at which asked "can regex do X?" and it made me wonder ...
196 views

### How should a website validate a users mailing address?

This is for a site that relies on shipping items via UPS or FedEx. I know there is software out there that does it (http://en.wikipedia.org/wiki/Coding_Accuracy_Support_System), but if you are trying ...
120 views

### What is the best strategy for transforming unicode strings into filenames?

I have a bunch (thousands) of resources in an RDF/XML file. I am writing a certain subset of the resources to files -- one file for each, and I'm using the resource's title property as the file name. ...
331 views

### Python flavor of regular expressions - related to which?

So my copy of the classic book, Mastering Regular Expressions, just arrived, and I'm scanning through it. The cover (third edition) says, "for Perl, PHP, Java, .NET, Ruby, and More!" Well, it does ...
298 views

### Why does Java define homogeneous methods in Matcher and Pattern classes

I wonder why java defines homogeneous methods in two distinct classes: java.util.regex.Pattern and java.util.regex.Matcher. For example split method defined in Pattern class and replaceAll method ...
867 views

### Lexical Analysis without regular expressions

I've been looking at a few lexers in various higher level langauges (Python, PHP, Javascript among others) and they all seem to use regular expressions in one form or another. While I'm sure regex's ...
11k views

### Is it a must for every programmer to learn regular expressions?

I am new to programming, and at an interview I got a question on regular expressions; needless to say I couldn't answer. So I was wondering whether I should learn regular expression? Is it a must for ...
5k views

### How do regular expressions actually work?

Say you have a document with an essay written. You want to parse this essay to only select certain words. Cool. Is using a regular expression faster than parsing the file line by line and word by ...
379 views

### Maintaining Regex in a .net application

I am sure everybody has had his/her share of regex woes. We have a big application with around 30 developers working on a mvc3 web application , and the two fold regex validations (front-end and ...
944 views

### Inspiring problems to show off the importance of regular expressions? [closed]

I am planning to give a presentation/demonstration on regular expressions at work to encourage young developers to add this powerful and important tool in their toolbox. Just teaching syntax doesn't ...
4k views

### When you should NOT use Regular Expressions?

Regular expressions are powerful tool in programmer's arsenal, but - there are some cases when they are not a best choice, or even outright harmful. Simple example #1 is parsing HTML with regexp - a ...
5k views

### How do you learn Regular Expressions? [closed]

I'm not asking where to learn. I've found lots of good resources online, and books etc. But how the heck do I tackle them. Where is the start of it, the end? When does the regexp processor advance on ...
274 views

### Documentation often ommits to specify which flavour of regular expression to use, so is there a default flavour that we should all be familiar with?

Often I come across documentation that says "use a regular expression here" I have to spend quite some time digging around trying to work out which regular expression format they are expecting. As ...
708 views

### What is more standard shortening for “regular expression”, “regex” or “regexp”? [closed]

I want to know what is more standard way to call regular expression in short way. I have seen both, regex and regexp. Google says regex has almost 2 as much hits that regexp, but it depends on way I ...
878 views

### Using lookahead assertions in regular expressions

I use regular expressions on a daily basis, as my daily work is 90% in Perl (legacy codebase, but that's a different issue). Despite this, I still find lookahead and lookbehind to be terribly ...
1k views

### A better way of doing Regex? [duplicate]

I really dislike regular expressions, each time I come back to it I seem to have to relearn it. It's also incredibly hard to maintain, modify and at a glance understand what it is doing. Has anyone ...
517 views

### What can / should be done with regular expressions?

I've recently learnt regular expressions and I love writing / using them. I'm looking for ideas and more opportunities to use them, however I don't want to overuse them as an all purpose tool, as ...