Matching Specific Characters

If I ask you whether these two words match, 0:00 you'd most likely glance at them both and say, hm, yes. 0:03 However, a regular expression doesn't work that way. 0:08 Instead, it uses a parser which compares each character in a regular 0:11 expression with a character in the string in the same position. 0:16 In other words, the regex parser requires two things. 0:20 An expression, also called a pattern, and a string to match. 0:23 The parser takes that pattern and walks through the string, 0:28 character by character. 0:31 First, it checks if there is a t. 0:33 Then if the t is followed by an o. 0:36 Which is followed by a y, then a b, o, 0:39 a, all the way to the t at the end. 0:43 At that point, the parser knows the string is a match. 0:47 I'm going to type toyboat into regexpal, in the regex pane, in the testing pane. 0:51 You can see there is a match selected here. 0:58 This is the basic matching we saw earlier. 1:01 Let's say we want to match the singular or the plural. 1:04 If I add an s to the regex, we lose our first match. 1:09 However, we can make this final s optional. 1:14 Match it if it exists but don't require it. 1:18 We can do that with another special regex character, the question mark. 1:21 The question mark matches the character that appears directly before it 1:25 zero times or one time. 1:29 In this case, the lower case s is before the question mark, so s is optional. 1:32 But if it does appear, it can only appear once. 1:38 So this regex will match toyboat or toyboats but not toyboatss with two S's. 1:41 Let's use the question mark in the middle of the string. 1:48 Let's match toybots, too. 1:50 We can put a question mark after the a to make it optional also. 1:53 It works. 1:57 I'll replace toybots with toyboats again, but this time with a space. 2:00 Just like before, 2:05 we can handle this with a question mark, making the space optional. 2:06 Now let's say that we want to match toy, even if it's capitalized. 2:14 Regular expressions are case sensitive by default, so 2:20 we'll need to specify that the T can be upper or lower case. 2:23 We can tell the interpreter to accept a capital or 2:28 lower case t by wrapping both characters in square brackets. 2:31 Let me show you what I mean. 2:36 I'll put a capital T next to the lowercase t, and surround both with square brackets. 2:38 I just added what is called a character set to the regex. 2:46 When the interpreter sees it, 2:50 exactly one of the characters in the set will match one position in the string. 2:51 We can specify as many characters as we want between these brackets. 2:57 Let's say we also want to match joy boats with a lowercase j, for example. 3:01 We can just put a j inside the square brackets. 3:06 There's two things to note. 3:10 No matter how many characters we put in the set, 3:11 the set will always match only one position of the string. 3:14 In this case, it's the first position. 3:18 It also makes no difference how you order the character set. 3:21 Now I wanted to give you a challenge. 3:24 I'll type out two more test cases for you. 3:26 See if you can modify the regex to match those 3:29 along with the strings it already matches. 3:32 I'll add a hyphenated version. 3:35 And another version with the B capitalized. 3:44 Go ahead and pause the video and see if you can do it. 3:48 For the hyphenated string, I replaced the space and 3:52 the regex with a character set containing a space and a hyphen. 3:55 Note that the question mark now indicates that the character set is optional. 4:02 In other words, the character between the y and the b can either be a space, 4:08 a hyphen, or nothing at all. 4:13 For the second string, 4:16 I used a character set to match either a lower or upper case B. 4:17 I've added some additional strings for 4:24 you to practice on in the teachers notes below. 4:26 Take some time to try then now if you'd like. 4:28 In the next video, I'll show you how to add a little more power to character sets. 4:32