These tools work on text as text: finding a pattern in it, comparing two versions, renaming an identifier, counting what’s there, or turning it into something that renders.
The regex tester runs a JavaScript pattern against your text and shows every match and group, the result of a replacement, and the pattern explained in plain English. The diff compares two texts by line, word or character. The case converter writes one phrase as camelCase, snake_case, kebab-case, Title Case and the rest, and splits acronyms like HTTPServer the way you’d expect. The word counter counts words, characters, sentences and paragraphs, with reading and speaking time, and the Markdown preview and table generator show what your Markdown will look like and write the tables for you.
The thing to know about regular expressions is that flavors differ. The tester uses the browser’s own JavaScript engine, so a pattern that works here may need changes for PCRE or Python’s re, which support possessive quantifiers and atomic groups that JavaScript doesn’t. With word counts, the definition of a word decides the number, so tools disagree on hyphens, URLs and contractions.
Three games train the same skills: the Regex Trainer teaches patterns one idea at a time, Regex Crossword trains you to read them, and Code Typing measures your speed on code, brackets and symbols included.