Merge pull request #81 from vsakarov/master

added regular expression section

Author: bbatsov

README.md

@@ -990,6 +990,67 @@ syntax.
     end
     ```
 
+## Regular Expressions
+
+* Don't use regular expressions if you just need plain text search in string:
+  `string['text']`
+* For simple constructions you can use regexp directly through string index.
+
+    ```Ruby
+    match = string[/regexp/]             # get content of matched regexp
+    first_group = string[/text(grp)/, 1] # get content of captured group
+    string[/text (grp)/, 1] = 'replace'  # string => 'text replace'
+    ```
+
+* Use non capturing groups when you don't use captured result of parenthesis.
+
+    ```Ruby
+    /(first|second)/   # bad
+    /(?:first|second)/ # good
+    ```
+
+* Avoid using $1-9 as it can be hard to track what they contain. Named groups
+  can be used instead.
+
+    ```Ruby
+    # bad
+    /(regexp)/ =~ string
+    ...
+    process $1
+
+    # good
+    /(?<meaningful_var>regexp)/ =~ string
+    ...
+    process meaningful_var
+    ```
+
+* Character classes have only few special characters you should care about:
+  `^`, `-`, `\`, `]`, so don't escape `.` or brackets in `[]`.
+
+* Be careful with `^` and `$` as they match start/end of line, not string endings.
+  If you want to match the whole string use: `\A` and `\Z`.
+
+    ```Ruby
+    string = "some injection\nusername"
+    string[/^username$/]   # matches
+    string[/\Ausername\Z/] # don't match
+    ```
+
+* Use `x` modifier for complex regexps. This makes them more readable and you
+  can add some useful comments. Just be careful as spaces are ignored.
+
+    ```Ruby
+    regexp = %r{
+      start         # some text
+      \s            # white space char
+      (group)       # first group
+      (?:alt1|alt2) # some alternation
+      end
+    }x
+    ```
+
+* For complex replacements `sub`/`gsub` can be used with block or hash.
+
 ## Percent Literals
 
 * Use `%w` freely.