Regex for PCRE, grep, ripgrep, and sed

grep -RInw 'error' ./logs

# Find full-word matches in files.

rg -n -i 'timeout|timed out' .

# Search recursively with ripgrep case-insensitively.

rg -n -P '(?<=ERROR: ).+' app.log

# Use lookbehind or advanced features through PCRE2 mode.

grep -RInv '^\s*(#|$)' ./config

# Show lines that do not match a pattern.

grep -E 'warn|error|fatal' app.log

# Search for one of several tokens.

sed -E 's/[[:space:]]+/ /g' file.txt

# Replace all runs of whitespace with a single space.

awk '/^[0-9]{4}-[0-9]{2}-[0-9]{2}/ { print }' file.txt

# Print lines matching a regex in awk.

(?>[A-Z][a-z]+)\s+\d+

# Avoid backtracking inside a grouped token.

^(?<pair>\((?:[^()]++|(?&pair))*\))$

# Reuse part of the pattern as a subroutine.

^\((?:[^()]++|(?R))*\)$

# Recursive pattern for balanced parentheses.

^(?:(<))?\w+(?(1)>)$

# Branch based on whether a group matched.

\b(?:https?://\S+)(*SKIP)(*F)|\bTODO\b

# Skip over matched prefixes before continuing.

sed -E 's/^([0-9]{4})-([0-9]{2})-([0-9]{2})$/\2\/\3\/\1/'

# Convert YYYY-MM-DD into MM/DD/YYYY.

sed -E 's/[[:space:]]+$//' file.txt

# Remove trailing whitespace from each line.

sed -E '/^$/! s/^/# /' notes.txt

# Add a prefix only to non-empty lines.

sed -En 's/.*"([^"]+)".*/\1/p' file.txt

# Print only the content inside double quotes on each line.