Created SC2101 (markdown)

SC2101.md

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+# Named class needs outer [], e.g. [[:digit:]].
+
+### Problematic code:
+
+    gzip file[:digit:]*.txt
+
+### Correct code:
+
+    gzip file[[:digit:]]*.txt
+
+### Rationale:
+
+Predefined character groups are supposed to be used inside character ranges. `[:digit:]` matches one of "digt:" just like `[abc]` matches one of "abc". `[[:digit:]]` matches a digit. 
+
+### Contraindications
+
+When passing an argument to `tr` which parses these by itself without relying on globbing, you should quote it instead, e.g. `tr -d '[:digit:]'`