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The Regex That Hangs Your Server at 3 a.m. — A Pre-Commit Guide
Five catastrophic-backtracking categories, four mechanical rewrites, and a 60-second pre-commit check that beats every ReDoS post-mortem.
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Batch Email Validation: From RFC 5321 Syntax to Real-World Deliverability
A batch email validator parses, normalises, and rates an entire list in one pass — and the gaps it leaves behind are exactly the gaps that matter when you decide which addresses to keep, quarantine, or drop.
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Why R-Squared Lies to You When You Add Predictors
Adjusted R-squared is the correction that penalizes each new variable for not pulling its weight. Use it to compare nested models instead of chasing raw R².
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Test Card Numbers That Actually Pass Luhn: A Credit Card Generator Walkthrough
BIN-correct, brand-correct, check-digit-valid test card numbers for Visa, MasterCard, Amex, Discover, JCB, and China UnionPay — without leaking real PII into your fixtures.
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VAT, GST, Sales Tax: The Calculator That Knows Which Number Is the Price
European VAT, Australian GST, and U.S. sales tax use the same five operations. The conventions are different. A working VAT calculator has to commit to inclusive or exclusive, round at the basket, and keep per-line data for the regulator.
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API Contract Stress Tester: Every Constraint Implies Its Own Negation
A boundary-value grammar that walks your OpenAPI document field by field — every documented constraint implies its own negation, and the tool turns that implication graph into a JSON test plan you can replay.
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Accessibility Audit Report Generator: 6 Categories, 4 Severities, 1 Readable PDF
A WCAG audit that ships a PDF a product manager can read in three minutes — 6 categories, 4 severity levels, 3 report sections, 1 shipped artifact.
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X.509 Certificate Decoder: 8 Fields, 4 Traps, 5 Sibling Tools for Reading PEM
Parse PEM certificates into subject, issuer, validity, SANs, fingerprint, and key usage — what openssl x509 prints, rendered without the command line.
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GeoJSON to KML Converter: The Two Map Formats That Don’t Speak the Same Language
GeoJSON lives in your browser, KML lives in desktop GIS. The translation rules between them are subtle — altitude, ExtendedData, and antimeridian crossings quietly lose fidelity if you get them wrong.
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RPE is the Intensity Scale Your Watch Can’t Read
A 1-to-10 number on every working set is the cheapest intensity measurement in training. Here's how to read it, log it, and stack it against heart rate and Training Stress Score.