Fix ziishaned#86 - Replace character set with character group

Author: ziishaned

README.md

@@ -405,13 +405,13 @@ that is matched by the first part of the expression. To define a positive
 lookahead, parentheses are used. Within those parentheses, a question mark with
 equal sign is used like this: `(?=...)`. Lookahead expression is written after
 the equal sign inside parentheses. For example, the regular expression
-`[T|t]he(?=\sfat)` means: optionally match lowercase letter `t` or uppercase
+`(T|t)he(?=\sfat)` means: optionally match lowercase letter `t` or uppercase
 letter `T`, followed by letter `h`, followed by letter `e`. In parentheses we
 define positive lookahead which tells regular expression engine to match `The`
 or `the` which are followed by the word `fat`.
 
 <pre>
-"[T|t]he(?=\sfat)" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat.
+"(T|t)he(?=\sfat)" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat.
 </pre>
 
 [Test the regular expression](https://regex101.com/r/IDDARt/1)
@@ -422,12 +422,12 @@ Negative lookahead is used when we need to get all matches from input string
 that are not followed by a pattern. Negative lookahead defined same as we define
 positive lookahead but the only difference is instead of equal `=` character we
 use negation `!` character i.e. `(?!...)`. Let's take a look at the following
-regular expression `[T|t]he(?!\sfat)` which means: get all `The` or `the` words
+regular expression `(T|t)he(?!\sfat)` which means: get all `The` or `the` words
 from input string that are not followed by the word `fat` precedes by a space
 character.
 
 <pre>
-"[T|t]he(?!\sfat)" => The fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat.
+"(T|t)he(?!\sfat)" => The fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat.
 </pre>
 
 [Test the regular expression](https://regex101.com/r/V32Npg/1)
@@ -436,11 +436,11 @@ character.
 
 Positive lookbehind is used to get all the matches that are preceded by a
 specific pattern. Positive lookbehind is denoted by `(?<=...)`. For example, the
-regular expression `(?<=[T|t]he\s)(fat|mat)` means: get all `fat` or `mat` words
+regular expression `(?<=(T|t)he\s)(fat|mat)` means: get all `fat` or `mat` words
 from input string that are after the word `The` or `the`.
 
 <pre>
-"(?<=[T|t]he\s)(fat|mat)" => The <a href="#learn-regex"><strong>fat</strong></a> cat sat on the <a href="#learn-regex"><strong>mat</strong></a>.
+"(?<=(T|t)he\s)(fat|mat)" => The <a href="#learn-regex"><strong>fat</strong></a> cat sat on the <a href="#learn-regex"><strong>mat</strong></a>.
 </pre>
 
 [Test the regular expression](https://regex101.com/r/avH165/1)
@@ -453,7 +453,7 @@ regular expression `(?<!(T|t)he\s)(cat)` means: get all `cat` words from input
 string that are not after the word `The` or `the`.
 
 <pre>
-"(?&lt;![T|t]he\s)(cat)" => The cat sat on <a href="#learn-regex"><strong>cat</strong></a>.
+"(?&lt;!(T|t)he\s)(cat)" => The cat sat on <a href="#learn-regex"><strong>cat</strong></a>.
 </pre>
 
 [Test the regular expression](https://regex101.com/r/8Efx5G/1)