Regex Meta Characters (with Examples)

A regular expression defines a search pattern for strings. This pattern may match one or several times or not at all for a given string. The abbreviation for regular expression is regex. Regular expressions can be used to search, edit, and manipulate text.

Regex meta characters are special symbols that carry a particular meaning and define the pattern in a regular expression. These characters allow us to create flexible and powerful search patterns for text processing.

1. List of Regex Meta Characters

Here’s a list of common regex meta-characters and their meanings:

Meta Character	Description
.	Matches any single character except a newline.
^	Matches the start of a string.
$	Matches the end of a string.
*	Matches zero or more repetitions of the preceding element.
+	Matches one or more repetitions of the preceding element.
?	Matches zero or one repetition of the preceding element (makes it optional).
{n}	Matches exactly n repetitions of the preceding element.
{n,}	Matches n or more repetitions of the preceding element.
{n,m}	Matches between n and m repetitions of the preceding element.
\	Escapes a meta-character, allowing it to be treated as a literal character.
[]	Matches any single character within the brackets.
[^]	Matches any single character not within the brackets.
()	Groups multiple tokens together and captures the matched text.
\d	Matches any digit, equivalent to [0-9].
\D	Matches any non-digit character, equivalent to [^0-9].
\w	Matches any word character (alphanumeric + underscore), equivalent to [a-zA-Z0-9_].
\W	Matches any non-word character, equivalent to [^a-zA-Z0-9_].
\s	Matches any whitespace character (spaces, tabs, line breaks).
\S	Matches any non-whitespace character.

2. Regex Meta Characters Example

Let us see a few examples of using the meta characters in regular expressions and matching them.

2.1. Dot (.) Meta Character

The dot meta-character matches any single character except for a newline (\n). It is useful to match a pattern where the character can be anything.

Pattern pattern = Pattern.compile(".at"); Matcher matcher = pattern.matcher("cat bat rat sat mat"); while (matcher.find()) { System.out.println(matcher.group()); // cat, bat, rat, sat, mat }

2.2. Caret (^) and Dollar ($) Meta Characters

The caret (^) matches the start of a string, and the dollar sign ($) matches the end of a string. These are used to ensure that the pattern matches the beginning or the end of the string, respectively.

// Matches "Hello" only if it is at the start of the string Pattern pattern = Pattern.compile("^Hello"); Matcher matcher = pattern.matcher("Hello world"); System.out.println(matcher.find()); // true

// Matches "world" only if it is at the end of the string pattern = Pattern.compile("world$"); matcher = pattern.matcher("Hello world"); System.out.println(matcher.find()); // true

2.3. Asterisk (*), Plus (+), and Question Mark (?) Meta Characters

• *: Matches zero or more repetitions of the preceding element.
• +: Matches one or more repetitions of the preceding element.
• ?: Matches zero or one repetition of the preceding element (makes it optional).

// Matches "a", "aa", "aaa", etc. Pattern pattern = Pattern.compile("a*"); Matcher matcher = pattern.matcher("aaab"); while (matcher.find()) { System.out.println(matcher.group()); // aaa } // Matches "a", "aa", "aaa", etc., but at least one "a" pattern = Pattern.compile("a+"); matcher = pattern.matcher("aaab"); while (matcher.find()) { System.out.println(matcher.group()); // aaa } // Matches "a" or "ab" pattern = Pattern.compile("ab?"); matcher = pattern.matcher("ab"); while (matcher.find()) { System.out.println(matcher.group()); // ab }

2.4. Braces ({}) Meta Characters

Braces are used to specify the exact number of repetitions for the preceding element.

• {n}: Exactly n repetitions.
• {n,}: At least n repetitions.
• {n,m}: Between n and m repetitions.

// Matches exactly 3 "a"s Pattern pattern = Pattern.compile("a{3}"); Matcher matcher = pattern.matcher("aaab"); while (matcher.find()) { System.out.println(matcher.group()); // aaa } // Matches 2 or more "a"s pattern = Pattern.compile("a{2,}"); matcher = pattern.matcher("aaaa"); while (matcher.find()) { System.out.println(matcher.group()); // aaaa } // Matches between 2 and 3 "a"s pattern = Pattern.compile("a{2,3}"); matcher = pattern.matcher("aaa"); while (matcher.find()) { System.out.println(matcher.group()); // aaa }

2.5. Square Brackets ([]) Meta Characters

Square brackets are used to define a character class, matching any single character within the brackets.

• [abc]: Matches any single character ‘a’, ‘b’, or ‘c’.
• [^abc]: Matches any single character except ‘a’, ‘b’, or ‘c’.

// Matches "a", "b", or "c" Pattern pattern = Pattern.compile("[abc]"); Matcher matcher = pattern.matcher("a1b2c3"); while (matcher.find()) { System.out.println(matcher.group()); // Matches 'a', 'b', 'c' } // Matches any character except "a", "b", or "c" pattern = Pattern.compile("[^abc]"); matcher = pattern.matcher("a1b2c3"); while (matcher.find()) { System.out.println(matcher.group()); // Matches '1', '2', '3' }

3. Escaping Meta Characters with Backslash (\)

A backslash is used to escape a meta-character, making it a literal character in the pattern. For example, \\. matches a literal dot (‘.’) character.

// Matches the literal dot character Pattern pattern = Pattern.compile("\\."); Matcher matcher = pattern.matcher("1.2.3"); while (matcher.find()) { System.out.println(matcher.group()); }

In this Java regex example, we learned to use meta characters in regular expressions to evaluate text strings.

Happy Learning !!