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+## Can't use `=~` in `[ ]`. Use `[[..]]` instead.
+
+### Problematic code:
+
+```sh
+[ "$input" =~ DOC[0-9]*\.txt ] && echo "match"
+```
+
+### Correct code:
+
+```sh
+[[ "$input" =~ DOC[0-9]*\.txt ]] && echo "match"
+```
+### Rationale:
+
+`=~` only works in `[[ .. ]]` tests. It does not work with `test` or `[` in any shell.
+
+If you're targeting POSIX `sh`, rewrite in terms of `case` or `grep` instead.
+
+### Exceptions:
+
+None.
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