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+## In POSIX sh, =~ regex matching is undefined.
+
+### Problematic code:
+
+```sh
+[ "$var" =~ .*foo[0-9]* ]
+```
+
+### Correct code:
+
+```sh
+expr "$var" : ".*foo[0-9]*" > /dev/null
+```
+
+### Rationale:
+
+You are using `=~` in a script declared to be compatible with POSIX sh or Dash.
+
+`=~` is not a POSIX operator and is unlikely to outside `[[ ]]` in Bash and Ksh.
+
+Use `expr`'s `:` operator instead.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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