Try some example, click on the links below and watch the Regex result:
FInd the names of models on the page https://en.wikipedia.org/wiki/Terminator_(character). Simplified.
1. model name - T- Find occurrences of this 2 literals.
2. model name - T-\d+ After that are one or more digits.
3. model name - T-(X|\d+) There are always some exceptions, the T-X model.
It also replaces T- with the word Robot().
4. XML - <title>(.+?)</title>  Find titles - not greedy.
5. XML - <year>(1[89]\d\d)</year>  Find old years.
6. XML - <year>(2\d{3})</year>  Find new years.
7. Just a simple email validation - ^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$
                        

//comment
                              println!("{}","one");
                        

//comment
                              println!("{}","one");
                        

//comment
                              println!("{}","one");
                        

//comment
                              println!("{}","one");
                        

    Basic Regex syntax
abc…      Letters literally
123…      Digits literally
[abc]     Only a, b, or c
[^abc]    Not a, b, nor c
[a-z]     Characters a to z
[0-9]     Numbers 0 to 9
\w        Any Alphanumeric character
\W        Any Non-alphanumeric character
\d        Any Digit
\D        Any Non-digit character
    Quantifiers:
?         Zero or one repetition
*         Zero or more repetitions
+         One or more repetitions
{m}       m Repetitions
{m,n}     m to n Repetitions

\s        Any Whitespace
\S        Any Non-whitespace character
.         Any Character (avoid using)
\.        Period - escaped
^…$       Starts and ends
    Capture groups:
(…)       Capture Group    
(?:…)     Non-Capture Group
(a(bc))   Capture Sub-group
(.*)      Capture all
(abc|def) Matches abc or def
    Substitution:
$1, $2    Captured group Substitution
    Flags:
(?m)    Multiple line matching (^$ -start/end of line)
(?i)    Case insensitive match
(?U)    Sets the engine to lazy matching - ungreedy

Avoid using .* dot (any character many times) because it is greedy. 
Instead use a negation of the next delimiter like [^/]*.
If needed, add Flags in the beginning like (?m) Multiple line matching.