Special Characters:
Dot . |
The dot . matches any single character including letters, digits, spaces, and so on. The only character it is unable to match with is the newline character\\n. |
|---|---|
Question mark ? |
Means that the preceding character may be present or absent. |
Backslash \\ |
Escape character, is used to separate key characters from ordinary ones. |
Set of chars [abc] |
Each set corresponds to a single character in the string, but what character exactly it can be is defined by the content of the set. The set is written in square brackets, []. For example, the set "[abc]" means that a single character "a", "b" or "c" can match it. |
Range of chars [0-9] |
You can specify the range designated by the dash symbol -. The character that precedes the dash denotes the starting point of the range, the character that follows it is the last character that falls into the range. [0-9] - this is a set of all digits from 0 to 9. Any value from the set can match. |
Excluding characters in sets [^abc] |
Allow you to define the set of chars that you don’t want. [^abc] - a, b, c - are excluded from permissible values. |
Excluding characters in ranges [^a-c] |
Allow you to define the range of chars that you don’t want. [^a-c] - a, b, c - are excluded from permissible values. |
| Alternations ` | ` |
Shorthands - pre-defined shorthands for the commonly used character sets:
\\d = [0-9] |
|
|---|---|
\\s = [\\t\\n\\x0B\\f\\r] |
a whitespace character (including tab and newline) |
\\w = [a-zA-Z_0-9] |
is an alphanumeric character (word) |
\\b |
is a word boundary. This one is a bit trickier: it doesn't match any specific character, it rather matches a boundary between an alphanumeric character and a non-alphanumeric character (for example, a white space character) or a boundary of the string (the end or the start of it). This way, "\\ba" matches all words (sequences of alphanumeric characters) starting with "a", "a\\b" matches all words ending with "a", and "\\ba\\b" matches all separate "a" preceded and followed by non-alphanumeric characters. |
\\D = [^0-9] |
is a non-digit |
\\S = [^ \\t\\n\\x0B\\f\\r] |
is a non-whitespace character |
\\W = [^a-zA-Z_0-9] |
is a non-alphanumeric character |
\\B |
is a non-word boundary. It 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. For example, "a\\B" matches all words that start with "a". |
Quantifiers - defines how often another character can occur in a regex pattern:
+ |
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. It is important not to include a space after the comma, otherwise it will not work. |
{n,} |
matches at least n repetitions of the preceding character |
{0,m} |
matches no more than m repetitions of the preceding character |
? or {0,1} |
makes the preceding character optional |
Additional useful links about regexp:
Other links: