Regex

Key resources

• Compose and testing expressions: https://regex101.com/
• Regex number range generator: Regex Numeric Range Generator — Regex Tools — 3Widgets

Shorthands for regex

Pre-defined shorthands

• \d is any digit, short for [0-9]
• \s is a whitespace character (including tab and newline), short for [ \t\n\x0B\f\r]
• \w is an alphanumeric character (word), short for [a-zA-Z_0-9]
• \b is a word boundary
  • A word boundary doesn’t match any specific character, it rather matches a boundary between an alphanumeric character and a non-alphanumeric character - for example, a whitespace character - or a boundary of the string (the end or the start of it) - examples:
    • \ba matches any sequence of alphanumeric characters that starts with “a”,
    • a\b matches any sequence of alphanumeric characters that ends with “a”
    • \ba\b matches a separate “a” preceded and followed by non-alphanumeric characters

Counterparts for shorthands

• \D is a non-digit, short for [^0-9]
• \S is a non-whitespace character, short for [^ \t\n\x0B\f\r]
• \W is a non-alphanumeric character, short for [^a-zA-Z_0-9]
• \B is a non-word boundary
  • A non-word boundary matches the situation opposite to that one of the \b shorthand: it finds its match every time whenever there is no “gap” between alphanumeric characters - examples:
    • a\B matches a part of a string that starts with “a” followed by any alphanumeric character which, in its turn, is not followed by a word or a string boundary

Any char

• . - any single character

Any preceeding char

• ? - the preceding character or nothing
• colou?r - colour or color but not colur or coloor
• ..? - there can be any single character or no character at all

Sets

• [] - groups of characters (to be placed inside the square brackets)
  • For example: [ab]x[12] matches “a” or “b” followed by “x” followed by either “1” or “2”.

Ranges

Use - inside a set to create a range. Java example:

String anyLetterPattern = "[a-z!?.A-Z]"; 
// matches any letter from a to z, A to Z, and any of the following: "!", "?", "."

Alternations

Use | to indicate an “or”. Java example:s

// Example 1:
String pattern = "yes|no|maybe"; 
// matches "yes", "no", or "maybe", but not "y" or "e"

// Example 2:
String pattern = "(b|r|go)at"; // matches "bat", "rat" or "goat"
String answer = "The answer is definitely (yes|no|maybe)";

Quantifiers

Here is a list of quantifiers to be remembered:

• + matches one or more repetitions of the preceding character;
• * matches zero or more repetitions of the preceding character;
• {n} matches exactly n repetitions of the preceding character;
• {n,m} matches at least n but not more than m repetitions of the preceding character;
• {n,} matches at least n repetitions of the preceding character;
• {0,m} matches no more than m repetitions of the preceding character.

Java examples:

// Example 1:
String johnRegex = ".*John.*"; 
// it matches all strings containing the substring "John"

// Example 2:
String regex = "1{2,3}"; 
// Note that the range specified in curly braces both starts and ends inclusively 
"1".matches(regex);    // false 
"11".matches(regex);   // true 
"111".matches(regex);  // true 
"1111".matches(regex); // false

Case insensitivity

You just need to add (?i) at the beginning of your regex. To make the dot character match the newline character, add (?s). You can enable both modes by writing (?is).

Backreference to match the same text again

\1 matches the exact same text that was matched by the first capturing group. For example, in (\b[a-z]+) /1 the \1 will match whatever (\b[a-z]+) had matched i.e. a repetition. This is useful to replace duplicate words.

Beginning or end of line

• ^ - the beginning of a line - example: ^[0-9]\s
• $ - the end of a line - example: ^[0-9]$

Negation / excluding characters

It is easy to define which characters are not wanted by writing the hat character ^ as the first one in the set.

• ^ - Any character, except the following

Capturing vs non-capturing groups

• (\w+) - group that captures any sequence of at least one alphanumeric character
• (?:\w)

Useful patterns

Email

^[a-zA-Z0-9_%+-.]+@[a-zA-Z]+\\.[a-zA-Z]+$

1. The e-mail username can consist of lowercase and uppercase Latin letters, digits and symbols _%+-.
2. The domain name (the part that comes after the @ symbol in an e-mail address) can consist only of lowercase and uppercase Latin letters
3. The e-mail domain should include a subdomain and a top-level domain

IP address

^((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}$

• Does not allow for leading zeros

UUID

^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$

• Is case insensitive

Regex in Java

Escape characters

• Use escape characters to protect a regex special symbol by putting the backslash \
• Note that when you want to use the backslash (\) itself in its literal meaning, you need to escape it as well - This way, a double backslash (\\) in your regex means a single backslash in the matching string

String pattern = ".....\\.";

"a1b2c.".matches(pattern); // true
"Wrong.".matches(pattern); // true
"Hello!".matches(pattern); // false

Pattern & Matcher in Java

Scanner scanner = new Scanner(System.in);
String stringWithNumbers = scanner.nextLine();
Pattern pattern = Pattern.compile("\\d{10,}");
Matcher matcher = pattern.matcher(stringWithNumbers);
while (matcher.find()) {
    System.out.println(matcher.group() + ":" + (matcher.end() - matcher.start()));
}