Regular Expressions

Regular Expressions

Assignment 2 was about a simplified version of Regular Expressions. Being curious, I studied a bit more on regexes. Regexes are mainly used in pattern matching with strings, or string matching ie. checking to see if a string matches the format of a valid email address.

Here, I want to provide a list of the most common meta-characters that are used in regexes:

• ".":  Despite what you might think after having completed assignment 2, the dot does not represent the joining of two expressions. It means any character except a new line ( a literal dot would be [.]). If you want to show the joining of two expressions, you just put them one after the other without anything in between.
• "( )": Groups a series of pattern elements to a single element.
• "+": Matches the element or pattern preceding it one or more times.
• "?": Matches the element or pattern preceding it zero or one times.
• "*": Matches the preceding pattern or element zero or more times.
• "{M,N}": Matches the preceding pattern or element at least M times and at most N times.
• "[...]": A set of possible character matches.
• "|": Alternate possibilities.
• "\b": I'm not sure what this one means. This is what Wikipedia is telling me, "Matches a zero-width boundary between a word-class character (see next) and either a non-word class character or an edge."
• "\w": Matches an alphanumeric character.
• "\W": Matches a non-alphanumeric character.
• "\s": Matches a whitespace character.
• "\S": Matches anything but a whitespace.
• "\d": Matches a digit.
• "\D": Matches a non-digit.
• "^": Matches the beginning of a line or string.
• "$": Matches the end of a line or string.
• "\A": Matches the beginning of a string.
• "\z": Matches the end of a string.
• "[^...]": Matches every character except the ones inside the brackets.

So, no you can understand what the following regex means:

• \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b

Lets go through this step by step:

1. Start of regex
2. Any alphanumeric character or one of the following: ., _, %, +, -
3. 2 repeated 1 or more times
4. The "@" character
5. Any alphanumeric character or the "-" character
6. The "." character
7. An alphabet
8. Step 7 repeated from 2 to 4 times
9. End of regex

As you might have guessed, this regex checks if a string has the format of a valid email address. WOOHOO! WE ROCK! :D