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[No Audio]

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Welcome back again in the course. In the

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previous tutorial, we have started our

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discussion about regexes. We will move ahead

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with the topic of regexes in this tutorial also.

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The next concepts in regexes is that of

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quantifiers. Quantifiers are used to denote

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the repetitions. There are three quantifiers

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that are commonly used called the question

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mark, a plus, and a star. The question mark is

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used for indicating zero or one times of

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repetition, the plus is used to indicate one

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or more times of repetition, and the star is

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used to indicate zero or more times of

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repetitions. So, let us learn the question

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mark first. I will write a simple Regex.

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[No Audio]

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This Regex means that, we need to have a zero or

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one instance of letter a at the beginning,

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followed by two mandatory letters of a.

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The question mark after the letter a means that

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the letter a is kind of optional at the

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beginning, that is zero or one times.

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Let us define some input text.

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[No Audio]

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I also include the code for

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displaying the possible matches.

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Let us cargo run this now.

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[No Audio]

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It has returned 2 matches,

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the first words, word matched because

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the first a is a kind of optional, and it is

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not present in the word. The second one also

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matched, because the first optional a is

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present in it. Let us do one more example,

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I will define a new Regex and also a new input text.

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[No Audio]

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In this case now, the Regex means that at the

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start, we need to have a mandatory letter of

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b followed by an optional a. In the input text,

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the first word will not match, because it does

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not have the mandatory letter of b.

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The second word containing ba will match.

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The next word containing only letter b will also

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match, because the letter a is option. Lastly,

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the last word containing ba will also match.

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Let us cargo run this.

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[No Audio]

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Let us now cover a nice use case

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of this quantifier, where you can use

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it to determine some files of a particular

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type, let me write a Regex.

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[No Audio]

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This Regex will match any text that contains

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the mandatory .rs at the end. The name of the

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file should have a word contains 0 letters,

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or at most 3 letters. Let us write some

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input text to test the Regex.

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[No Audio]

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In this case, it will match the first two

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file names, but the third one since the length

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of the fourth file name is greater than three

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characters, will not match.

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Let us cargo run this to confirm.

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[No Audio]

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The next quantifier is that

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of the plus, which is used to indicate one or

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more times. Let us comment out the

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previous code and start fresh.

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[No Audio]

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I will define a simple Regex.

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[No Audio]

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This Regex means that we need to have

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one or more times of letter a in the input

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text for a match to occur. Let us add some input text.

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[No Audio]

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I will also add the code for

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displaying the matches. This will now match

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the first word containing a single a, the

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second word containing double a, and the third

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word containing aaa. However for the

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word baab, it will only match the couple of

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a's, and from the last word, it will only match

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a single a. Let us execute to confirm.

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[No Audio]

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To find a file which may contain any number

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of characters, but has a .gif extension, we will

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use a Regex which will look something like this.

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[No Audio]

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This Regex will match anything that

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starts with any number of characters from one

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onwards, but that ends on the letters of

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.gif. The slash before the dot is used to

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make sure that the dot is used in its literal

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meaning and not for making any type of

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character. Let me add some text to test this Regex.

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[No Audio]

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Let us execute the code now to see the results.

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[No Audio]

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The next quantifier is that of the

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star, which is used to indicate zero or more

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times of repetitions. For instance I will

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write a simple Regex, which will match any

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text that starts with a mandatory letter of

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a followed by zero or more times of letter b.

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[No Audio]

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Let us add a sample text to test.

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[No Audio]

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In this case, it will match all the three

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words in the text, let us cargo run this to confirm.

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[No Audio]

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Let us now move to the next topic, but before

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that, I will comment out the previous code

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and start fresh again.

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[No Audio]

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Sometimes, we may do not

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want to have unlimited number of repetitions,

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but rather we are interested in some limited

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repetitions. To mention the limited

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repetitions, we will mention the least and

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most number of times of repetitions inside

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the curly brackets, let us use it inside the Regex.

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[No Audio]

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This Regex will match any text

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containing between three to five number of

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characters,  let us add a simple text.

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[No Audio]

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Let us also add the code for displaying the matches.

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Let us cargo run this now.

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[No Audio]

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You may note that, it only matches the words

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that are between the given range of characters.

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For those words who are greater than length,

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only their first five characters have

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matched. You may use the word boundary meta

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corrector that is slash be at the start enter

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at the end of the Regex to enforce to only

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matches the words, and do not return partial

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word from the text. Let me add and cargo run again.

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[No Audio]

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You may note that, the partial words are not

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being returned now. A nice use case of

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limited repetition is, to limit the number of

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digits in the fraction and whole number part

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of a number. For instance, let me add a Regex.

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[No Audio]

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This Regex will make sure that the number

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of digits in the whole and fraction parts are

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between one to three digits with a dot

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between the two parts. Let me add some text for testing.

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[No Audio]

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Let us cargo run this now.

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[No Audio]

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Again to exclude the last

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number and make sure that, only that

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it only returns the valid numbers, we will use

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the word boundary at the beginning

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and at the end of the Regex.

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[No Audio]

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Now the last number is

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being excluded. Sometimes we may know the

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least number of times, but we may do not know

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the maximum number of times the repetitions

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will occur. In that case, we will only mention

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the first number in the curly brackets and

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we'll keep the second number empty, let me add a Regex.

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[No Audio]

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This will match any text containing three or

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more digits. Let us also add some text for testing.

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[No Audio]

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In this case, it will only match the first

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three numbers, since they contain three or

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more digit, the last number will not be

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matched. Lastly, we can mention a fixed

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number of times of repetitions by placing a

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single number within the curly brackets, this

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is quite simple and will be covered in the

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next example. Let us now move to the next

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topic, which is capturing groups, I will

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comment out the code first.

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[No Audio]

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Sometimes the pattern we want to define may

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be a bit more complex, and therefore we may

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want to break it down into its individual

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components for the sake of simplification.

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This can be done by placing parts of a

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regular expression inside parentheses to

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create groups or parts within a regular

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expression. Consider a case for detecting the

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dates which are given in the form of year,

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month, and day. The year is in the form of

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four digits, followed by a month containing

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two digits, and lastly the day containing two

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digits. We will write a Regex for each part

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of the year, day, and month separately using

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the capturing groups. The Regex is going to

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be simple, so let us write it and then we will explain.

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[No Audio]

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The Regex in the first smooth brackets will

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match the year. The year contains exactly four

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digits, so inside the curly brackets we wrote

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four, which will match exactly four digits.

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Next we have a mandatory dash, this is

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followed by another group which will contain

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a couple of digits for the months, followed by

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another mandatory dash, and finally another

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group for the day containing two digits. Let

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us write some dates in the input text now.

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[No Audio]

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Lastly, we will add the code for displaying

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the result. The value at the index zero of

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the capture will contain the full match, and

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the individual groups will be in the

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remaining of the indexes. In particular, the

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value at the index 1 will contain the year

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group, the index 2 will contain the value

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for the month, and the index 3 will contain

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the value for the day. Let us add a suitable

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print statement to display the result.

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[No Audio]

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Let us go go run this now.

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[No Audio]

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With this we end

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this tutorial. There are many details of Regexes,

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but we intentionally leave them for now

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for the sake of briefness. Since this is a

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Rust course and not a regular expression

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course, therefore we will limit it to these

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details. For those of you who are interested

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in learning more about Regexes, they may take

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our other course on regular expression, you

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can ask for a coupon of that and I will give

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you away a free coupon for the course. For now

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the details covered in these tutorials are

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enough to give you a quick start and build

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simple regular expressions. See you again and

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until next tutorial, enjoy Rust programming.

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[No Audio]