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Regex Replace Examples Cheat Sheet

Apply regex-based replacements for cleanup, normalization, and data transformation in common scripting workflows.

What this cheat sheet is for

This regex replace examples reference helps you complete common developer tasks quickly and avoid repeated trial-and-error when working under deadlines.

  • When you need a reliable command or pattern during debugging and incident response.
  • When you are implementing a feature and want a known-good syntax example immediately.
  • When onboarding teammates and documenting repeatable workflows for daily use.
  • When reviewing pull requests and verifying that command usage follows best practices.
  • Normalize whitespace in imported data.
  • Reformat dates and identifiers in logs.
  • Remove sensitive tokens before sharing files.

Basic replacement patterns

Replace all matches globally

"foo foo".replace(/foo/g, "bar")

Example:

sed -E "s/[0-9]{4}-[0-9]{2}-[0-9]{2}/DATE/g" file.txt

Case-insensitive replacement

text.replace(/error/gi, "WARN")

Example:

perl -pe "s/error/WARN/gi" app.log

Capture groups in replacement

Swap first and last name

"Ada Lovelace".replace(/(\w+) (\w+)/, "$2, $1")

Example:

sed -E "s/(.*),(.*)/\2-\1/" values.txt

Reformat date

"2026-02-01".replace(/(\d{4})-(\d{2})-(\d{2})/, "$3/$2/$1")

Example:

python -c "import re;print(re.sub(r"(\d{3})-(\d{2})-(\d{4})", r"***-**-\3", "123-45-6789"))"

Conditional and scoped replace

Replace at word boundaries only

text.replace(/\bcat\b/g, "dog")

Example:

text.replace(/\s+/g, " ").trim()

Replace only first occurrence

text.replace(/id=/, "ID=")

Example:

text.replace(/:/, " -> ")

Common mistakes / Pitfalls

  • Forgetting global flag g replaces only the first match in many languages.
  • Backreference syntax differs between tools ($1 vs \1), causing wrong output.
  • Greedy capture groups can swallow too much text during replacement.

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Last updated: February 2026