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            <h3>RegEx</h3>
            <p>A RegEx(Regular Expression) is a sequence of characters that forms a search pattern. It can be used to check if a string contains the specified search pattern.
<h4>RegEx Module</h4>
Python has a built-in package called re, which can be used to work with Regular Expressions. <br>
<b>Import the re module:</b>
<pre><code>
</code></pre>
When you have imported the  <b>re</b> module, you can start using   <b>regular expressions</b>: <br>
<b>Example:</b>
<pre><code>
#Check if the string starts with "The" and ends with "Spain":
txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)
  print("YES! We have a match!")
  print("No match")
</code></pre>
<b>Output:</b>
<pre><code>
YES! We have a match!
</code></pre>
<h4>RegEx Functions</h4>
<th>Function</th>
<th>Description</th>
<td>findall</td>
<td>Returns a list containing all matches</td>
<td>search</td>
<td>Returns a Match object if there is a match anywhere in the string</td>
<td>split</td>
<td>Returns a list where the string has been split at each match </td>
<td>sub</td>
<td>Replaces one or many matches with a string</td>
<h4>Metacharacters</h4>
<th>Character</th>
<th>Description</th>
<th>Example</th>
<td>[]</td>
<td>A set of characters</td>
<td>&quot;[a-m]&quot;</td>
<td>Signals a special sequence (can also be used to escape special characters)</td>
<td>&quot;\d&quot;</td>
<td>Any character (except newline character)</td>
<td>&quot;he..o&quot;</td>
<td>Starts with</td>
<td>&quot;^hello&quot;</td>
<td>Ends with</td>
<td>&quot;world$&quot;</td>
<td>Zero or more occurrences</td>
<td>&quot;aix*&quot;</td>
<td>One or more occurrences</td>
<td>&quot;aix+&quot;</td>
<td>{}</td>
<td>Exactly the specified number of occurrences</td>
<td>&quot;al{2}&quot;</td>
<td>Either or</td>
<td>&quot;falls|stays&quot;</td>
<td>()</td>
<td>Capture and group</td>
<td>&nbsp;</td>
<h4>Special Sequences</h4>
<th>Character</th>
<th>Description</th>
<th>Example</th>
<td>\A</td>
<td>Returns a match if the specified characters are at the beginning of the 
string</td>
<td>&quot;\AThe&quot;</td>
<td>\b</td>
<td>Returns a match where the specified characters are at the beginning or at the 
end of a word<br>(the &quot;r&quot; in the beginning is making sure that the string is 
being treated as a &quot;raw string&quot;)</td>
<td>r&quot;\bain&quot;<br>r&quot;ain\b&quot;</td>
<td>\B</td>
<td>Returns a match where the specified characters are present, but NOT at the beginning 
the end) of a word<br>(the &quot;r&quot; in the beginning is making sure that the string 
is being treated as a &quot;raw string&quot;)</td>
<td>r&quot;\Bain&quot;<br>r&quot;ain\B&quot;</td>
<td>\d</td>
<td>Returns a match where the string contains digits (numbers from 0-9)</td>
<td>&quot;\d&quot;</td>
<td>\D</td>
<td>Returns a match where the string DOES NOT contain digits</td>
<td>&quot;\D&quot;</td>
<td>\s</td>
<td>Returns a match where the string contains a white space character</td>
<td>&quot;\s&quot;</td>
<td>\S</td>
<td>Returns a match where the string DOES NOT contain a white space character</td>
<td>&quot;\S&quot;</td>
<td>\w</td>
<td>Returns a match where the string contains any word characters (characters from 
a to Z, digits from 0-9, and the underscore _ character)</td>
<td>&quot;\w&quot;</td>
<td>\W</td>
<td>Returns a match where the string DOES NOT contain any word characters</td>
<td>&quot;\W&quot;</td>
<td>\Z</td>
<td>Returns a match if the specified characters are at the end of the string</td>
<td>&quot;Spain\Z&quot;</td>
<h4>Sets</h4>
<th>Set</th>
<th>Description</th>
<td>[arn]</td> class="w3-codespan">a</code>,
<code>r</code>, or <code>n</code>) are 
present</td>
<td>Returns a match where one of the specified characters (<code>a</code>,
<code>r</code>, or <code>n</code>) are 
present</td>
<td>[a-n]</td>
<td>Returns a match for any lower case character, alphabetically between
<code>a</code> and <code>n</code></td>
<td>[^arn]</td>
<td>Returns a match for any character EXCEPT <code>a</code>,
<code>r</code>, and <code>n</code></td>
<td>[0123]</td>
<td>Returns a match where any of the specified digits (<code>0</code>,
<code>1</code>, <code>2</code>, or <code>
3</code>) are 
present</td>
<td>[0-9]</td>
<td>Returns a match for any digit between
<code>0</code> and <code>9</code></td>
<td>[0-5][0-9]</td>
<td>Returns a match for any two-digit numbers from <code>00</code> and <code >
59</code></td>
<td>[a-zA-Z]</td>
<td>Returns a match for any character alphabetically between
<code>a</code> and <code>z</code>, lower case OR upper case</td>
<td>[+]</td>
<td>In sets, <code>+</code>, <code>*</code>,
<code>.</code>, <code>|</code>,
<code>()</code>, <code>$</code>,<code>{}</code> 
has no special meaning, so <code>[+]</code> means: return a match for any
<code>+</code> character in the string</td>
<h4>findall() Function</h4>
<p>findall() function returns a list containing all matches</p>
<b>Example <br>
Print a list of all matches:</b>
<pre><code>
#Return a list containing every occurrence of "ai":
txt = "The rain in Spain"
x = re.findall("ai", txt)
</code></pre>
<b>Output:</b>
<pre><code>
['ai', 'ai']
</code></pre>
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned
<h4>The search() Function
<p>The search() function searches the string for a match, and returns a Match object if there is a match.
If there is more than one match, only the first occurrence of the match will be returned
<br><b>Example:</b>
<pre><code>
txt = "The rain in Spain"
x = re.search("\s", txt)
print("The first white-space character is located in position:", x.start()) 
</code></pre>
<b>Output:</b>
<pre><code>
The first white-space character is located in position: 3
</code></pre>
If no matches are found, the value None is returned</p>
<h4>split() Function</h4>
<p>The split() function returns a list where the string has been split at each match <br>
<b>Example</b>
<pre><code>
#Split the string at every white-space character:
txt = "The rain in Spain"
x = re.split("\s", txt)
</code></pre></p>
<b>Output:</b>
<pre><code>
['The', 'rain', 'in', 'Spain']
</code></pre>
            <hr>
            <h4>sub() Function</h4>
<p>The sub() function replaces the matches with the text of your choice
<b>Example:</b>
<pre><code>
#Replace all white-space characters with the digit "9":
txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
</code></pre></p>
<b>Output:</b>
<pre><code>
The9rain9in9Spain
</code></pre>
<h4>Match Object</h4>
<p>A Match Object is an object containing information about the search and the result. <br>
Note: If there is no match, the value None will be returned, instead of the Match Object. <br>
<b>Example</b>
<pre><code>
#The search() function returns a Match object:
txt = "The rain in Spain"
x = re.search("ai", txt)
</code></pre>
<b>Output:</b>
<pre><code>
<_sre.SRE_Match object; span=(5, 7), match='ai'>
</code></pre>
The Match object has properties and methods used to retrieve information about the search, and the result: <br>
.span() returns a tuple containing the start-, and end positions of the match. <br>
.string returns the string passed into the function <br>
.group() returns the part of the string where there was a match</p>
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