Characters and Metacharacters in Regular Expressions
Characters
In regular expressions, characters are the basic building blocks. They represent literal characters that you want to match in the text. For example, the character 'a' in a regular expression will match the letter 'a' in the text.
Example: The regular expression cat will match the string "cat" in the text "The cat sat on the mat."
Metacharacters
Metacharacters are special characters that have specific meanings in regular expressions. They are used to define more complex patterns. Some common metacharacters include ., *, +, ?, ^, $, \, [, ], {, }, (, and ).
Example: The metacharacter . (dot) matches any single character except a newline. The regular expression c.t will match "cat", "cot", "cut", etc., in the text "The cat sat on the mat."
Common Metacharacters and Their Meanings
.(dot): Matches any single character except a newline.*: Matches zero or more occurrences of the preceding element.+: Matches one or more occurrences of the preceding element.?: Matches zero or one occurrence of the preceding element.^: Matches the start of a line.$: Matches the end of a line.\: Escapes a metacharacter, making it a literal character.[ ]: Defines a character class, matching any one of the enclosed characters.{ }: Specifies the exact number of occurrences of the preceding element.( ): Groups elements together.
Examples of Metacharacters in Action
1. a*: Matches zero or more 'a's. It will match "", "a", "aa", "aaa", etc.
2. a+: Matches one or more 'a's. It will match "a", "aa", "aaa", etc., but not "".
3. a?: Matches zero or one 'a'. It will match "", "a".
4. ^start: Matches "start" only if it appears at the beginning of a line.
5. end$: Matches "end" only if it appears at the end of a line.
6. \.: Matches a literal dot. The backslash escapes the dot, making it a literal character.
7. [aeiou]: Matches any one of the vowels 'a', 'e', 'i', 'o', 'u'.
8. a{3}: Matches exactly three 'a's. It will match "aaa" but not "a", "aa", or "aaaa".
9. (abc)+: Matches one or more occurrences of the sequence "abc". It will match "abc", "abcabc", "abcabcabc", etc.