Basic Regular Expression Concepts

1. Metacharacters

Metacharacters are special symbols in regular expressions that have unique meanings. They allow you to define patterns more precisely. Some common metacharacters include:

. - Matches any single character except a newline.
^ - Matches the start of a string.
$ - Matches the end of a string.
* - Matches zero or more occurrences of the preceding element.
+ - Matches one or more occurrences of the preceding element.
? - Matches zero or one occurrence of the preceding element.

Example:

To match a string that starts with "Hello" and ends with "world", you can use the following regular expression:

^Hello.*world$

This pattern will match strings like "Hello there world" or "Hello world".

2. Character Classes

Character classes allow you to match any one of a specific set of characters. They are enclosed in square brackets []. Some common character classes include:

[abc] - Matches any one of the characters 'a', 'b', or 'c'.
[0-9] - Matches any digit from 0 to 9.
[A-Z] - Matches any uppercase letter from A to Z.
[a-z] - Matches any lowercase letter from a to z.
[^0-9] - Matches any character that is not a digit.

Example:

To match a string that contains only digits, you can use the following regular expression:

^[0-9]+$

This pattern will match strings like "12345" or "987654321".