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+         
+         <p>Raw link: <a href="https://www.youtube.com/watch?v=TxYGHjKBMUg">https://www.youtube.com/watch?v=TxYGHjKBMUg</a></p>
+         
+         <p>In this video tutorial I show how to use regular expression syntax to
+solve various practical problems in Emacs.</p>
+
+<p>Knowledge of regexp notation is not a prerequisite to using Emacs
+effectively.  In fact, you can be very productive without knowing
+anything about regular expressions.  However, knowing those things
+will certainly boost your productivity and make Emacs an even more
+powerful tool at your hands.</p>
+
+<p>See <a href="https://protesilaos.com/emacs/dotemacs">my dotemacs</a> for the
+documentation and package declarations I provide.</p>
+
+<hr />
+
+<p>This is the full text of my presentation, which was done using
+<code>org-mode</code> (check my dotemacs for presentations with Org).</p>
+
+<pre><code>* Emacs regular expressions in practice
+
+Emacs has a few ways to operate on regexp matches, such as:
+
++ =isearch=
++ =query-replace=
++ =keep-lines=
++ =flush-lines=
+
+To make our life easier, we can practice with the built-in
+=regexp-builder= or the third-party package =visual-regexp=.  This demo
+will rely on the latter.
+
+If you have the manual you can run =C-h r i regexp= to get to the
+relevant chapter.  *Do it!*
+
+** Line boundaries
+
+The caret =^= denotes the beginning of the line.
+
+The dollar sign =$= marks the end.
+
+Match all lines that start with a space:
+
+Emacs
+Emacs
+ Emacs
+Emacs
+ Emacs
+
+And all that end with a capital =S=:
+
+emacs emacS
+emacS emacs
+emacs emacs
+emacS emacS
+
+** Remove or keep lines
+
+Remove the empty lines.  Then keep the ones that contain "username".
+
+&lt;username&gt;&lt;![CDATA[name]]&gt;&lt;/username&gt;
+emacs emacS
+emacS emacs
+emacs emacs
+emacS emacS
+
+
+
+&lt;userName&gt;&lt;![CDATA[nom]]&gt;&lt;/userName&gt;
+emacs emacS
+emacS emacs
+emacs emacs
+emacS emacS
+
+
+
+
+&lt;username&gt;&lt;![CDATA[name]]&gt;&lt;/username&gt;
+emacs emacS
+emacS emacs
+emacs emacs
+emacS emacS
+
+** The dot character
+
+The dot or full stop =.= means matches every character except the
+newline.
+
+Match these words using their common part =ired= as a string.
+
+dired
+fired
+mired
+tired
+wired
+
+** Character sets and ranges
+
+A set of individual characters is marked between brackets =[]=.
+
+Sets can be written as ranges:
+
+| Range      | Scope                                      |
+|------------+--------------------------------------------|
+| [a-z]      | all lower cases alphabetic characters      |
+| [A-Za-z]   | all upper or lower case letters            |
+| [a-z0-9]   | lower case alphabet or numbers 0 through 9 |
+| [abcd1234] | letters a,b,c,d and numbers 1,2,3,4        |
+
+Match both of those using a character set for the first letter:
+
+emacs
+Emacs
+
+Match those that end with a number:
+
+Emacs
+emacs-27
+emacs-26
+GNU emacs
+
+** Difference between postfix operators ?, +, *
+
+"Postfix" means that it comes after a given set and alters its scope.
+
+=?= match the previous term zero or one time.
+=+= match the previous term one or more times.
+=*= match the previous term zero or as many times as possible.
+
+Match the =s= optionally:
+
+day
+days
+
+Use =prote= followed by a postfix:
+
+prot
+prote
+proteeee
+
+** Grouped matches
+
+A group is enclosed inside escaped parentheses =\(GROUP\)=.
+
+Match both of these, including the optional suffix =ig=:
+
+conf
+config
+
+** Greedy versus non-greedy
+
+Postfix charaacter are greedy by default.  "Greedy" matches the
+longest possible part.  Whereas "non-greedy" corresponds to the
+shortest.
+
+A non-greedy variant is used when the postfix is followed by =?=.
+
+Using the =.*= construct, match items both greedily and not:
+
+Hello world
+Hello world world world world
+
+** Multiple groups
+
+Match the alphabetic and numeric parts in two separate groups.
+
+emacs27
+emacs26
+emacs25
+emacs24
+
+** Literal hyphen and dot
+
+Match the hyphen as part of the alphabetic group and the dot as part
+of the numeric one.
+
+emacs-27.1
+emacs-26.3
+emacs-25.2
+
+** Exclude sets
+
+To exclude a set you prepend a caret sign: =[^SET]=
+
+Match every line except those that start with a capital letter.
+
+GNU
+Emacs
+org-mode
+regexp
+emacs_lisp
+Linux
+guix
+
+** Alternative groups with literal brackets
+
+Use a character sets that matches =name= and =nom=.
+
+name
+nom
+
+Then:
+
+1. Match the =username= variants' =[name]= or =[nom]=.
+2. Replace the match with =[PROT]=.
+
+
+&lt;username&gt;&lt;![CDATA[name]]&gt;&lt;/username&gt;
+&lt;nameuser&gt;&lt;![CDATA[nam]]&gt;&lt;/nameuser&gt;
+&lt;userName&gt;&lt;![CDATA[nom]]&gt;&lt;/userName&gt;
+&lt;nameuser&gt;&lt;![CDATA[nome]]&gt;&lt;/nameuser&gt;
+
+** Either match
+
+To target either set, use =\|=.
+
+Prepend =vr/= to the first =group= and =match= on each line.
+
+`(group-0 ((group (:inherit modus-theme-intense-blue))))
+`(group-1 ((group (:inherit modus-theme-intense-magenta))))
+`(group-2 ((group (:inherit modus-theme-intense-green))))
+`(match-0 ((match (:inherit modus-theme-refine-yellow))))
+`(match-1 ((match (:inherit modus-theme-refine-yellow))))
+
+** Running elisp functions on groups
+
+Run elisp by escaping the comma =\,= and then following it with a symbol
+inside parentheses: =\,(FUNCTION)=.
+
+Using the =.ired= pattern from earlier, run a replace command where you
+must execute the =upcase= function on the second/middle match.  Keep the
+rest in tact.
+
+direddireddired
+firedfiredfired
+miredmiredmired
+tiredtiredtired
+wiredwiredwired
+</code></pre>
+        
+