Solr treats these characters with a special meaning when they are used in a query:

+  -  &&  ||  !  ( )  { }  [ ]  ^  "  ~  *  ?  :  /

Using a backslash character() before the special character will notify Solr not to treat it as a special character but as a normal character. For example, for the search string (1+1):2 the plus and parentheses, and colon can be ignored as special characters and will be treated as normal characters like this:

(1+1):2