Regular Expressions Basics

The pattern language inside every text tool

Regex appears in grep, sed, awk, vim, your editor's find dialog, and every modern programming language. The dialects vary; the core grammar is stable.

Building blocks

• Any character: .
• Character class: [abc] = a, b, or c. [^abc] = anything but. [a-z] = range.
• Predefined: \d digit (PCRE), \w word, \s whitespace.
• Anchors: ^ start of line, $ end. \b word boundary (PCRE).

Quantifiers

• * — 0 or more.
• + — 1 or more (ERE/PCRE).
• ? — 0 or 1 (ERE/PCRE).
• {3} — exactly 3.
• {2,5} — 2 to 5.

Grouping and alternation

• (foo|bar) — foo or bar.
• (\d+) — capture digits, refer back as \1.

BRE vs ERE vs PCRE

• BRE (basic, default for grep/sed) — +, ?, {}, and () need backslashes.
• ERE (extended, grep -E, sed -E) — the modern, sane dialect.
• PCRE (Perl-compatible, grep -P) — full power: lookahead, lookbehind, named groups.

Practical patterns

• Email-ish: [A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}
• IP-ish: \b(\d{1,3}\.){3}\d{1,3}\b
• ISO date: \d{4}-\d{2}-\d{2}
• Trailing whitespace: [[:space:]]+$

Real email validation is a nightmare; that pattern is just a strong filter. Don't try to write a perfect regex for human languages.