How do you use grep with regular expressions?

grep PATTERN file prints lines matching the pattern; key flags are -i (ignore case), -v (invert), -n (line numbers), -r (recurse), -E (extended regex).

The name comes from the old ed editor command g/re/p — "globally search for a regular expression and print." Each flag bends its behaviour: -v flips it to "show lines that don't match" (great for filtering out comments), -c counts instead of printing, -r walks a directory tree. Chain them with pipes for quick log triage.

Common options:

Examples:

# Find "error" (case insensitive)
grep -i "error" logfile.txt

# Show lines NOT matching pattern
grep -v "^#" config.file

# Remove comments and empty lines
grep -v "^[#;]" file.txt | grep -v "^$"

# Count matches
grep -c "pattern" file.txt

# Search recursively in directory
grep -r "TODO" /project/src/

Filter /etc/ethertypes (remove comments):

grep -v '^[#;]' /etc/ethertypes

Go deeper:

• grep(1) man page (man7.org) — -i -v -n -c -r -E and the BRE/ERE behaviour.