Regex Basics for Fail2ban Filters | Lesson 07 of 13

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Fail2ban filters are regular expressions in a tiny dialect with one extension: the HOST placeholder. This lesson is the regex foundation — character classes, quantifiers, anchors (^.+ on every regex, always), greedy vs lazy matching, and reading sshd.conf line by line. Once you can read this, you can read any Fail2ban filter ever written.

This is lesson 07 of the 13-lesson Fail2ban Mastery course by Dargslan — a complete production-grade training that takes you from zero to running Fail2ban at scale across a server fleet.

What you'll learn in this lesson

✅ The HOST placeholder — the only Fail2ban-specific token
✅ ^.+ anchor: why every filter needs it
✅ 5 character classes + 4 quantifiers cover 95% of cases

Course structure

🔴 Stage 01 — Foundations (Lessons 00-03): Mental model, install, log scanning
🟡 Stage 02 — Jails (Lessons 04-06): Configuration, tuning, multi-jail coordination
🔵 Stage 03 — Filters & Actions (Lessons 07-09): Regex, custom filters, action stacking
🟢 Stage 04 — Production Operations (Lessons 10-12): Monitoring, backup, capstone

Timestamps

00:00 The HOST placeholder
02:00 ^.+ anchor — always
03:30 Character classes
04:00 Quantifiers + greedy/lazy
06:00 Reading sshd.conf annotated

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