Steps to reproduce:

1. Go to Administration→General
2. Select Icon mapping
3. Click Create icon map
4. Enter some silly Name
5. Enter / in Expression field
6. Click Add button

Result:
See invalid_regular_expression.png

Cannot create icon map
Invalid parameter "/1/mappings/1/expression": invalid regular expression. [adm.iconmapping.php:70 ? CApiWrapper->__call() ? CFrontendApiWrapper->callMethod() ? CApiWrapper->callMethod() ? CFrontendApiWrapper->callClientMethod() ? CLocalApiClient->callMethod() ? CIconMap->create() ? CIconMap->validateCreate() ? CApiService::exception() in include/classes/api/services/CIconMap.php:192]

Expected:
Icon map is successfully created because / is a valid regular expression (check it here), forward slash is not a special character.

Comments:
Even though forward slash is not a special character it needs to be escaped in the pattern if forward slash is also used as delimiter (which is a very common practice). Unfortunately, preg_quote() cannot be used here because we don't want to escape special characters user wants to provide in his/her pattern. The best approach would probably be to surround user pattern with parentheses, brackets or braces as suggested in this wonderful article.