Regular expressions are an incredibly useful tool for processing text data. They allow you to create a search pattern that actually matches many different text strings. Thus instead of having to write out each and every word and phrase with all the types of capitalization and punctuation, you can instead write a single regular expression to find them all at once. Regular expressions or regexes reads text from left to right. If it matches the first character, then it evaluates the second character. If it doesn't match, then it starts over, moves one character to the right and re-evaluates the first character. This continues until all characters of the regular expression are matched. Part of the reason that regular expressions are so flexible is that you can use wildcards and character classes to match any one of many characters at once. In regular expression, the period is a wildcard that can match any single character. Be it a letter, a number or whitespace. Things like spaces, tabs, and new lines. Character classes are collections of characters that can match any one of the values in the character class. For example, if you want to match any letter in the alphabet, you can use brackets to indicate any letter A through Z. Note that this is case-sensitive. You can do the same thing for capital A through capital Z, and if you want to match any letter regardless of the case, you can combine these into a single bracket. If you don't want to match the characters in a bracket, you can put a curly at the beginning of the character class. In some cases, there are popular groupings that actually get a character class shorthand. For example, backslash W matches all alphanumeric characters. Well, backslash D matches all digits. There are also character classes that exclude types of matches. For example, backslash W matches all non alphanumeric characters. You can also use quantifiers after the character or character class to say how many times you want to match that particular character or character class. For example, if you want it to match at least once, you can use the plus sign. If you want it to match exactly twice, you can use curly brace too. Use the same notation for matching anywhere between two and four times. Finally, if the character or character classes option, you can use the asterisks to map zero or more times. If you wanted to only match zero or one times, you can use a question mark. If you want to allow for matching one value or another, you can use a vertical bar to alternate. The bar matches either the character before the bar or after the bar. You can also use parentheses to group values with your whole statement if you want to choose between groups of characters. Parentheses also allow you to create capturing groups. Capturing groups basically save everything that matches the regular expression in the parenthesis. You can then extract this information if you want to in another command. Other searches you may want to do may require you to only look for matches at the beginning or the end of the string. This is called anchoring. The curly can be used at the beginning of the regular expression to only match the beginning of the input while the dollar sign will only match the end of the input. Regular expressions also have modifiers that can apply. The G modifier is the global command. It will keep searching the tax and find all matches rather than just stopping after the regular expression finds the first match. The I modify our makes the regular expression case insensitive, meaning that it will match uppercase and lowercase letters. The actual software driving and executing these regular expressions are called regex engines. Two of the most common regex engines are PCRE, Perl Compatible Regular Expressions and JavaScript regular expressions. Unfortunately, these two regex engines each have their own rules and have slightly different character classes. Packages and commands in "I" use a variety of regular expression engines. In general, when a regular expression isn't working as you think it should, make sure to check which engine you are using. I can't tell you how much time I have lost trying to figure out why my awesome regular expression wasn't working only to find out that it was written using a different engine. At this point in the video, your head is probably spinning and how much content we have covered so quickly. That's okay and totally expected. Regular expressions are extremely complicated and really require you to just practice applying them to truly get to know them inside and out. The good news is, that there are a number of resources on the web to help you learn regular expressions. Two of my favorite web tools are RegExr and Debuggex. Both tools support PCRE and JavaScript regular expressions. They allow you to copy and paste your own text to use while building your regular expressions. However, they have different ways of displaying information about the regular expression that you may find helpful. Let's take this example. I wanted to build a regular expression that can match any of these phrases. In RegExr, I can see that the regular expression I built does match all of the phrases because they all turn blue. Down in the tool section, I can see more details about each component of the regular expression. On the left side, are a number of references and tools you can use to help develop your RegExr. Overall, I think that this is the best tool for checking what does and does not match a RegExr. However, I love Debuggex for understanding and building your regular expression because it actually diagrams the logic of the regular expression. Let's take the exact same RegExr. As we can see at group 1, there's the option to just skip the whole part if necessary. So, the first phrase we wanted to match physical exam colon, matches the first part then skips group 1 and group 2 before matching the colon and completing. If we change the regular expression to match group 1, zero or more times, now there is also a loop. That means we can match the nation as many times as we want. We will link to these resources in the radiance and give you lots of examples to help you get started. With some practice and hard work, I have no doubt that you'll be able to apply regular expressions in no time.
