Regular Expression Characterization?

1. a dollar sign ($) is used to match strings that end with the given pattern.
2. a caret (^) character at the beginning of a regular expression indicates that it must match the beginning of the string.

1. The dot (.) metacharacter matches any single character except newline (\).
2. The vertical pipe (|) metacharacter is used for alternatives in a regular expression.

The metacharacters +, * and ? affect the number of times a pattern should be matched.

1. '+' means Match one or more of the preceding expression
2. '*' means Match zero or more of the preceding expression
3. '?' means Match zero or one of the preceding expression

Curly braces {} can be used differently

1. With a single integer, {n} means match exactly n occurrences of the preceding expression
2. With one integer and a comma, {n,} means match n or more occurrences of the preceding expression
3. With two comma-separated integers {n,m} means match the previous character if it occurs at least n times, but no more than m times

Regular Express with Respective Matches

1. foo -------> Matches three letters or four numbersThe string "foo"
2. ^foo -------> "foo" at the start of a string
3. foo$ -------> "foo" at the end of a string
4. ^foo$ -------> "foo" when it is alone on a string
5. [abc] -------> a, b, or c
6. [a-z] -------> Any lowercase letter
7. [^A-Z] -------> Any character that is not a uppercase letter
8. (gif|jpg) -------> Matches either "gif" or "jpeg"
9. [a-z]+ -------> One or more lowercase letters
10. [0-9\.\-] -------> Аny number, dot, or minus sign
11. ^[a-zA-Z0-9_]{1,}$ -------> Any word of at least one letter, number or _
12. ([wx])([yz]) -------> wy, wz, xy, or xz
13. [^A-Za-z0-9] -------> Any symbol (not a number or a letter)
14. ([A-Z]{3}|[0-9]{4}) -------> Matches three letters or four numbers