Assertions

Basic Characters:

Expression  Explanations ^ Matches the expression to its right, at the start of a string before it experiences a line break $ Matches the expression to its left, at the end of a string before it experiences a line break . Matches any character except newline a Matches exactly one character a xy Matches the string xy a|b Matches expression a or b. If a is matched first, b is left untried....

Quantifiers:

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Character Classes:

Expressions Explanations + Matches the expression to its left 1 or more times. * Matches the expression to its left 0 or more times. ? Matches the expression to its left 0 or 1 times {p} Matches the expression to its left p times, and not less. {p, q} Matches the expression to its left p to q times, and not less. {p, } Matches the expression to its left p or more times. { , q} Matches the expression to its left up to q times...

Sets:

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Groups:

Expressions Explanations \w  Matches alphanumeric characters, that is a-z, A-Z, 0-9, and underscore(_) \W Matches non-alphanumeric characters, that is except a-z, A-Z, 0-9 and _ \d Matches digits, from 0-9. \D  Matches any non-digits. \s Matches whitespace characters, which also include the \t, \n, \r, and space characters. \S Matches non-whitespace characters. \A Matches the expression to its right at the absolute start of a string whether in single or multi-line mode. \Z  Matches the expression to its left at the absolute end of a string whether in single or multi-line mode. \n Matches a newline character \t Matches tab character \b Matches the word boundary (or empty string) at the start and end of a word. \B Matches where \b does not, that is, non-word boundary...

Assertions:

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Flags:

Expressions Explanations [abc]  Matches either a, b, or c. It does not match abc. [a-z] Matches any alphabet from a to z. [A-Z] Matches any alphabets in capital from A to Z [a\-p] Matches a, -, or p. It matches – because \ escapes it. [-z] Matches – or z [a-z0-9] Matches characters from a to z or from 0 to 9. [(+*)]  Special characters become literal inside a set, so this matches (, +, *, or ) [^ab5]  Adding ^ excludes any character in the set. Here, it matches characters that are not a, b, or 5. \[a\] Matches [a] because both parentheses [ ] are escaped...

Conclusion

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