Traps

macOS vs GNU sed — in-place edit syntax

# WRONG — macOS requires explicit empty string for no backup
sed -i 's/old/new/' /tmp/file.txt          # Works on GNU sed (Linux)

# CORRECT — portable across macOS and GNU
sed -i'' 's/old/new/' /tmp/file.txt         # macOS requires -i ''
sed -i.bak 's/old/new/' /tmp/file.txt       # Safe on both — always creates backup

Greedy .* matching — captures too much

# WRONG — greedy .* eats through the first bracket pair
echo '[ERROR] [2026-04-11] disk full' | sed 's/\[.*\]/REDACTED/'
# Output: REDACTED disk full  (both bracket pairs consumed)

# CORRECT — use negated character class
echo '[ERROR] [2026-04-11] disk full' | sed 's/\[[^]]*\]/REDACTED/'
# Output: REDACTED [2026-04-11] disk full  (first pair only)

Slash in pattern — use alternate delimiter

# WRONG — escaping slashes is unreadable
echo '/usr/local/bin/python3' | sed 's/\/usr\/local\/bin/\/opt\/bin/'

# CORRECT — use | or # as delimiter
echo '/usr/local/bin/python3' | sed 's|/usr/local/bin|/opt/bin|'
echo '/usr/local/bin/python3' | sed 's#/usr/local/bin#/opt/bin#'

Newline in replacement — requires literal newline or escape

# WRONG — \n in replacement does not work in all sed versions
echo 'a;b;c' | sed 's/;/\n/g'

# CORRECT — use escaped literal newline (works everywhere)
echo 'a;b;c' | sed 's/;/\
/g'

Single quotes inside sed expression

# WRONG — cannot nest single quotes
echo "it's here" | sed 's/it's/it was/'

# CORRECT — end quote, escape, restart quote
echo "it's here" | sed 's/it'\''s/it was/'

# CORRECT — use double quotes (watch for shell expansion)
echo "it's here" | sed "s/it's/it was/"

BRE vs ERE — capture group syntax differs

# BRE (default) — escape parentheses
echo 'port=443' | sed 's/port=\([0-9]*\)/PORT:\1/'

# ERE (-E flag) — bare parentheses
echo 'port=443' | sed -E 's/port=([0-9]+)/PORT:\1/'

BRE vs ERE — quantifiers differ

# WRONG — + is literal in BRE
echo 'aaa' | sed 's/a+/X/'
# Output: aaa  (no match — + is literal)

# CORRECT — use -E for extended regex
echo 'aaa' | sed -E 's/a+/X/'
# Output: X

Hold buffer — confusing multi-line state

# Swap adjacent lines using hold buffer — easy to get wrong
cat <<'EOF' > /tmp/pairs.txt
key1
value1
key2
value2
EOF
# h = copy to hold, n = next line, G = append hold
# This reverses each pair: value first, then key
sed 'h;n;G' /tmp/pairs.txt

Empty regex reuses previous pattern — surprising behavior

# The empty // reuses the last regex
echo 'hello world' | sed -e 's/hello/goodbye/' -e 's//farewell/'
# Second s// reuses /hello/ — but it already changed, so no match
# This is confusing — always be explicit
echo 'hello world' | sed -e 's/hello/goodbye/' -e 's/goodbye/farewell/'