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+# tr replaces sets of chars, not words (mentioned due to duplicates).
+
+### Problematic code:
+
+    echo 'hello world' | tr 'hello' 'goodbye'
+
+### Correct code:
+
+    echo 'hello world' | sed -e 's/hello/goodbye'
+
+### Rationale:
+
+`tr` is for `tr`ansliteration, turning some characters into other characters. It doesn't match strings or words, only individual characters. 
+
+In this case, it transliterates h->g, e->o, l->d, o->y, resulting in the string "goddb wbrdd" instead of "goodbye world".
+
+The solution is to use a tool that does string search and replace, such as sed. 
+
+### Contraindications
+
+None. 
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