In this Regular expression tutorial, learn to match a single character appearing once, a single character appearing multiple times, or a specific set of characters.
In regular expressions:
- To match any character, use the dot
"."pattern. - To match a single character (or multiple characters) zero or more times, use
".*"pattern. - To match multiple characters or a given set of characters, use the character classes.
1. Match Any Character
By default, the '.' dot character in a regular expression matches a single character without regard to what character it is. The matched character can be an alphabet, a number or, any special character.
To create more meaningful patterns, we can combine the dot character with other regular expression constructs.
| Pattern | Description |
|---|---|
. (Dot) | Matches only a single character. |
A.B | Matches only a single character at second place in a 3 character long string where the string starts with ‘A’ and ends with ‘B’. |
Pattern.compile(".").matcher("a").matches(); //true
Pattern.compile(".").matcher("ab").matches(); //false
Pattern.compile("A.B").matcher("AIB").matches(); //true
Pattern.compile("A.B").matcher("ABI").matches(); //false
Pattern.compile("A[abc]B").matcher("AaB").matches(); //true
Pattern.compile("A[abc]B").matcher("AkB").matches(); //false2. Match Any Character from the Specified Range
If we want to match a range of characters at any place, we need to use character classes with a hyphen between the ranges. e.g. ‘[a-f]’ will match a single character which can be either of ‘a’, ‘b’, ‘c’, ‘d’, ‘e’ or ‘f’.
| Pattern | Description |
|---|---|
[abc] | Matches only a single character from a set of given characters. |
[aA] | Matches only a single character ‘a’, case-insensitive. |
[a-f] | Matches only a single character in the range from ‘a’ to ‘f’. |
[a-z] | Matches only a single lowercase character in the range from ‘a’ to ‘z’. |
[A-Z] | Matches only a single uppercase character in the range from ‘A’ to ‘Z’. |
[a-zA-Z] | Matches only a single character in the range from ‘a’ to ‘z’, case-insensitive. |
[0-9] | Matches only a single number in the range from ‘0’ to ‘9’. |
Pattern.compile("[a-f]").matcher("b").matches(); //true
Pattern.compile("[a-f]").matcher("g").matches(); //false
Pattern.compile("[a-zA-Z]").matcher("a").matches(); //true
Pattern.compile("[a-zA-Z]").matcher("B").matches(); //true
Pattern.compile("[a-zA-Z]").matcher("4").matches(); //false
Pattern.compile("[0-9]").matcher("9").matches(); //true
Pattern.compile("[0-9]").matcher("91").matches(); //false3. Match Any Character: Zero or More Occurrences
The asterisk (*) is used with any regex pattern for matching zero or more occurrences within strings.
| Pattern | Description |
|---|---|
.* | Matches any number of characters including special characters. |
[0-9]* | Matches any number of digits. |
[a-zA-Z]* | Matches any number of alphabets. |
[a-zA-Z0-9]* | Matches any number of alphanumeric characters. |
https?://.*?(?:\s|$) | Matches URLs from Text |
\(.*\) | Matches text between parentheses |
Pattern.compile(".*").matcher("abcd").matches(); //true
Pattern.compile("[a-zA-Z]*").matcher("abcd").matches(); //true
Pattern.compile("[0-9]*").matcher("01234").matches(); //true
Pattern.compile("[a-zA-Z0-9]*").matcher("a1b2c3").matches(); //trueHappy Learning !!
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