fix special pattern syntax for grep

source/Unix/search_filter.rst

@@ -70,13 +70,13 @@ grep with Regular Expressions
     * **[]** bracket expressions: defines a set of characters
         * *[abc]* : matches character *"a"* or *"b"* or *"c"*
         * *[a-z]* : matches on character in the range from *"a"* to *"z"*, ie *"a"* or *"b"* ... or *"z"*
-        * *[:alnum:]* : matches any letter or numberic characters
-        * *[:alpha:]* : matches any letter
-        * *[:digit:]* : matches any number
-        * *[:lower:]* : matches any letter in range [a-z]
-        * *[:upper:]* : matches any letter in range [A-Z]
-        * *[:punct:]* : matches any punctuation character
-        * *[:space:]* : matches any space character (space, tab, ...)
+        * *[[:alnum:]]* : matches any letter or numberic characters
+        * *[[:alpha:]]* : matches any letter
+        * *[[:digit:]]* : matches any number
+        * *[[:lower:]]* : matches any letter in range [a-z]
+        * *[[:upper:]]* : matches any letter in range [A-Z]
+        * *[[:punct:]]* : matches any punctuation character
+        * *[[:space:]]* : matches any space character (space, tab, ...)
     * Anchoring.
         * **^** represent start of the string.
         * **$** represent end of the string.