--- Anchors ---

^ Start of line/string $ End of line/string \b Word boundary \B Non-word boundary --- Character Classes --- . Any character except newline \d Digit [0-9] \D Non-digit \w Word character [a-zA-Z0-9_] \W Non-word character \s Whitespace (space, tab, newline) \S Non-whitespace [abc] Any of a, b, or c [^abc] Any character NOT in a, b, c [a-z] Character range [a-zA-Z0-9] Alphanumeric POSIX classes (for grep, sed without -E): [[:alpha:]] Letters [[:digit:]] Digits [[:alnum:]] Letters and digits [[:space:]] Whitespace [[:upper:]] Uppercase letters [[:lower:]] Lowercase letters [[:punct:]] Punctuation --- Quantifiers --- * 0 or more + 1 or more (ERE/PCRE only; use + in BRE) ? 0 or 1 (ERE/PCRE only; use \? in BRE) {n} Exactly n {n,} n or more {n,m} Between n and m (inclusive) ? Non-greedy 0 or more (PCRE only) +? Non-greedy 1 or more (PCRE only) --- Groups & Alternation --- (abc) Capture group (?:abc) Non-capture group (PCRE) a|b Alternation (a or b) \1 Back-reference to group 1 --- Lookahead / Lookbehind (PCRE only) --- (?=...) Positive lookahead (?!...) Negative lookahead (?<=...) Positive lookbehind (?<!...) Negative lookbehind --- Common Patterns --- Email address: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,} IPv4 address: \b([0-9]{1,3}.){3}[0-9]{1,3}\b URL: https?://[^\s/$.?#].[\s] Semantic version (semver): v?[0-9]+.[0-9]+.[0-9]+ Hex color:

([0-9a-fA-F]{3}|[0-9a-fA-F]{6})¶

ISO date (YYYY-MM-DD): [0-9]{4}-[0-9]{2}-[0-9]{2} UUID: [0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12} YAML front matter block: ^---\n[\s\S]*?\n--- --- Tool Usage --- grep (BRE by default, -E for ERE, -P for PCRE):

grep '<pattern>' <file>
grep -E '<ere-pattern>' <file>
grep -P '<pcre-pattern>' <file>

rg (ripgrep, PCRE2 by default)

rg '<pattern>' <path>
rg -P '<pcre-pattern>' <path>       # explicit PCRE2
rg '<pattern>' --type py <path>     # filter by file type

sed (BRE by default, -E for ERE)

sed 's/<pattern>/<replacement>/' <file>
sed -E 's/<ere-pattern>/<replacement>/' <file>

awk

awk '/<pattern>/ { print $0 }' <file>

To test a regex interactively (requires perl)

echo "test string" | perl -ne 'print if /<pattern>/'