Regular Expressions

Advanced~30 min read

Master regex with grep -E and sed -E to search, extract, and transform text streams efficiently.

grep -E and sed -E

Output

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Output

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Note: grep -E enables EREs; grep -P (PCRE) may not be available everywhere.

Pro Tip: Use -o to extract only matches and -n to show line numbers.

Caution: Some shells alias grep; prefer explicit flags (-E, -F) for clarity.

# Wrong (\d is not ERE)
grep -E '\\d+' file
# Correct (use [0-9] or -P if available)
grep -E '[0-9]+' file

# Over-matching
echo 'a[b]c[d]e' | sed -E 's/\[[^]]*\]/X/g'

Task: Read lines from stdin and replace the username of emails with ***, keeping the domain.

Output

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Show Solution

sed -E 's/[A-Za-z0-9._%+-]+@/***@/g'

Edit streams powerfully with sed.