Pattern Matching: Regex & Extended Globs

In Plain English

Pattern matching in Bash operates on two distinct engines: Shell Pathname Expansion (Globbing like `*.txt`, `[0-9]`, `{a,b}`) and Regular Expressions (Regex like `^`, `$`, `[a-z]+`, `(foo|bar)`). Mastering both empowers advanced filtering and file manipulation.

The Mental Model

"Globs are wide fishing nets designed to scoop up files by name; Regexes are precision surgical scalpels designed to match detailed string patterns inside text."

Deep Dive: How It Works

Basic vs Extended Regex: BRE (`grep`) requires escaping special chars like `$`, `$`, `\+`; ERE (`grep -E` or `egrep`) supports raw `+`, `?`, `|`, `()` directly.

Regex Anchors: `^` matches start of line, `$` matches end of line, `\b` matches word boundary.

Bash extglob: `shopt -s extglob` enables `?()`, `*()`, `+()`, `@()`, and `!()` inverse globs.

Syntax Blueprint

grep -E "^[0-9]{4}-[0-9]{2}-[0-9]{2}" log.txt  # ISO date at line start
grep -E "(ERROR|FATAL|PANIC)" app.log          # Multiple keywords

Use grep -E with anchors and quantifier groups for advanced pattern matching.

Core Rules to Remember

Anchors ^ and $: ^ matches the start of a line and $ matches the end.

Quantifiers: + means 1 or more, * means 0 or more, ? means 0 or 1, and {n,m} specifies exact repetition ranges.

Character Classes: [0-9] or [[:digit:]], [a-zA-Z] or [[:alpha:]], [[:alnum:]], [[:space:]].

Common Beginner Traps & How to Fix Them

Mistake:Using unquoted regex metacharacters like `*` or `?` directly in shell commands.

Why it happens: Bash expands globs before passing arguments to the command.

How to fix: Always wrap regex patterns in single quotes: `grep -E '^[a-z]+'`.

Pro Tip: Use `grep -oP` with Perl-compatible regex to extract only the matching substring rather than the entire line.

Live Interactive Example

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Matching Email Patterns with ERE

Git 2.45 / CLI

Output Console

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How it works: The extended regex matched lines containing valid RFC-compliant email formats.

Your Turn: Micro Challenge

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Micro Exercise

Match Lines Starting with Numbers

Create `/tmp/regex_test.txt` with lines "123 Main St", "First Ave", and "456 Oak Rd".
Use `grep -E "^[0-9]+"` to output lines starting with numbers.
Delete the temporary file.

Sandbox Output

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