Regex New Added 15 October 2026

sed: -e expression #1, char 15: unknown option to `s'

sed ended the s command early because the pattern or replacement contained the delimiter, nearly always a / inside a path or URL. Everything after that slash is read as flags, and the first character that is not a valid flag triggers the error. It typically appears when a shell variable is substituted into the expression, so the same script works until a value with a slash comes along.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# The failing shape: the variable contains slashes
NEW=/opt/app/bin
sed -i "s/^PATH_DIR=.*/PATH_DIR=$NEW/" app.conf   # unknown option to `s'

# Use a delimiter that cannot appear in the value
sed -i "s|^PATH_DIR=.*|PATH_DIR=$NEW|" app.conf

# Or escape the delimiter and & in the value first
SAFE=$(printf '%s' "$NEW" | sed 's/[\/&]/\\&/g')
sed -i "s/^PATH_DIR=.*/PATH_DIR=$SAFE/" app.conf

# See exactly what sed receives
echo "s/^PATH_DIR=.*/PATH_DIR=$NEW/"

How to diagnose Regex errors

Regex problems come in two flavours: it does not match what you expect (usually escaping, greediness, or an engine feature difference) and it matches but takes forever. The second is catastrophic backtracking, a genuine denial-of-service vector known as ReDoS, caused by nested quantifiers over overlapping character classes, such as (a+)+b. Any regex applied to untrusted input should be checked for it.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Test against a visualiser that shows backtracking steps, and always test with input that fails to match. That is where catastrophic backtracking appears, not on successful matches.
Eliminate nested quantifiers over overlapping classes. Rewrite (a+)+ as a+, and prefer possessive quantifiers or atomic groups where the engine supports them.
Know your engine: lookbehind, named groups and Unicode property escapes differ between PCRE, RE2, JavaScript, Python and Go. RE2 (used by Go) deliberately has no backtracking and rejects some patterns outright.
Anchor patterns with ^ and $ where you mean a whole-string match. Unanchored patterns silently match substrings.
Set a timeout or use a linear-time engine for user-supplied patterns. Never run an untrusted regex on a request thread.

Tools worth reaching for

regex101.com
re2 / RE2J
Python re.DEBUG
grep -P for PCRE testing
recheck / redos linters

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

gnu.org

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