Regex Cheat Sheet

• ^ start of string, $ end of string.
• . matches any character, .* means zero or more of any character.
• \d digit, \w word char, \s whitespace.
• [abc] one character from set, [^abc] negation.
• () grouping, | OR pattern.
• Escape special characters when used literally.

^abc$              # exact abc
^\d+$              # digits only
^[A-Za-z]+$        # letters only
^[A-Za-z0-9_]+$    # simple username
.+@.+\..+          # simple email
(FAIL|ERROR|WARN)  # one of these words

Pattern p = Pattern.compile("^\d+$");