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wtf is rseq?

alexeygrigorev/rseq — explained in plain English

Analysis updated 2026-07-14 · repo last pushed 2016-11-25

1JavaAudience · developerComplexity · 2/5DormantSetup · easy

TL;DR

Rseq is a Java library for pattern-matching over sequences of objects, bringing regular-expression-like searching to lists of Java objects instead of just strings.

Mindmap

mindmap
  root((repo))
    What it does
      Pattern matching for lists
      Regex-like for objects
      Capture matched portions
    Tech stack
      Java
    Use cases
      Extract definitions from text
      Parse ordered token lists
      Find structures in sequences
    Features
      Optional elements
      Grouping and variables
      Repeating elements support
    Audience
      Java developers
      Data processing tasks

Code map

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filefunction / class

Why would anyone build with this?

REASON 1

Extract definitions from a list of words by matching a word, the word 'is', an optional article, and the definition.

REASON 2

Search ordered token lists for specific combinations or structures without writing custom loops.

REASON 3

Capture portions of a matched sequence for later use in your Java application.

What's in the stack?

Java

How it stacks up

alexeygrigorev/rseqalexeygrigorev/codeforces-solutions-javacoderonak/modular-monolithic-cloud-file-storage-backend
Stars111
LanguageJavaJavaJava
Last pushed2016-11-252020-10-03
MaintenanceDormantDormant
Setup difficultyeasyeasyhard
Complexity2/51/54/5
Audiencedeveloperdeveloperdeveloper

Figures from each repo's GitHub metadata at analysis time.

How do you spin it up?

Difficulty · easy Time to first run · 30min

Java library that requires understanding the tutorial for writing custom matchers, as the README does not cover it in detail.

Wtf does this do

Rseq is a Java library that lets you search through lists of objects the way you'd search through text with a regular expression. Instead of being limited to scanning strings character by character, you can define a pattern and find matches in any sequence of Java objects. This is useful when you have ordered data and need to spot specific combinations or structures within it. At a high level, you build a pattern by chaining together matching rules. For example, you can tell the library to look for a specific item, optionally skip a word like "the" or "a," and then capture whatever comes next so you can use it later. Once you define this sequence of rules, the library scans your list and identifies where those conditions are met. It also handles common Java data structures directly and provides ways to save or capture portions of the match for later use. A practical use case would be processing a list of words or tokens from a sentence. If you want to automatically extract definitions, you could write a pattern that looks for a word, then the word "is," then an optional "the" or "a," and finally the definition itself. Instead of writing custom loops and logic to parse the sentence, the library handles the matching for you. Beyond simple matching, the library supports variables, grouping, optional elements, and a version of the Kleene star, which allows for repeating elements. The README doesn't go into detail on how to write these custom matchers, but it points to a separate tutorial for those looking to explore further.

Yoink these prompts

Prompt 1
Help me write a rseq pattern in Java that finds a word, then 'is', then an optional 'the' or 'a', and captures the rest of the sequence as a definition.
Prompt 2
How do I use rseq to match repeating elements in a list of Java objects, similar to a Kleene star in regex?
Prompt 3
Show me how to chain matching rules in rseq to skip optional items and capture specific portions of a matched sequence for later use.

Frequently asked questions

wtf is rseq?

Rseq is a Java library for pattern-matching over sequences of objects, bringing regular-expression-like searching to lists of Java objects instead of just strings.

What language is rseq written in?

Mainly Java. The stack also includes Java.

Is rseq actively maintained?

Dormant — no commits in 2+ years (last push 2016-11-25).

How hard is rseq to set up?

Setup difficulty is rated easy, with roughly 30min to a first successful run.

Who is rseq for?

Mainly developer.

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