Created SC2305 (markdown)

SC2305.md

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+## Quote regex argument to expr to avoid it expanding as a glob.
+
+### Problematic code:
+
+```sh
+expr "$input" : [0-9]*
+```
+
+### Correct code:
+
+```sh
+expr "$input" : "[0-9]*"
+```
+
+### Rationale:
+
+ShellCheck found an `expr` command using `:` to match a regex, but the regex is not quoted and therefore being treated as a glob.
+
+This means that if the problematic code is ever executed in a directory containing a file matching `[0-9]*`, such as `2021-reports` or `12 Angry Men [1957].mkv`, it will be replaced be replaced and cause the command to error or incorrectly match.
+
+The regex should be quoted to avoid this, like in the correct example.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!