116 lines
2.5 KiB
Markdown
116 lines
2.5 KiB
Markdown
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---
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obj: concept
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aliases: ["Regular Expression"]
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wiki: https://en.wikipedia.org/wiki/Regular_expression
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---
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# Regex
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A regular expression (shortened as regex or regexp), is a sequence of characters that specifies a match pattern in text. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation.
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## Anchors
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### `^`
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Matches the beginning of the string or line.
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Example: `^word`
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### `$`
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Matches the end of the string or line.
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Example: `\.txt$`
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## Flags
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- `i`: Makes the expression case insensitive
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- `g`: Ensures that the expression does not stop on the first match
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## Group & References
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### `()`
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Groups an expression.
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Example: `(ha)+`
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### `\1`
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References a grouped expression. `\1` references the first group, `\2` the second and so on.
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Example: `(ha)\s\1`
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### `(?:)`
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Makes a grouping that cannot be referenced.
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Example: `(?:ha)+`
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## Character Classes
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### `[abc]`
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Matches any character in the set.
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Example: `b[eo]r`
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### `[^abc]`
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Matches any character not in the set.
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Example: `b[^eo]r`
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### `[a-z]`
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Matches all characters between two characters, including themselves.
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Example: `[e-i]`
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### `.`
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Matches any character except line breaks.
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### `\w`
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Matches any alphanumeric character. Including the underline.
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### `\W`
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Matches any non-alphanumeric character.
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### `\d`
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Matches any numeric character.
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### `\D`
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Matches any non-numeric character.
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### `\s`
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Matches any whitespace character.
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### `\S`
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Matches any non-whitespace character.
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## Lookarounds
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### `(?=)`
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Positive Lookahead.
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Example: `\d(?=after)`
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### `(?!)`
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Negative Lookahead.
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Example: `\d(?!after)`
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### `(?<=)`
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Positive Lookbehind.
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Example: `(?<=behind)\d`
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### `(?<!)`
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Negative Lookbehind.
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Example: `(?<!behind)\d`
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## Quantifiers And Alternation
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### `+`
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Expression matches one or more.
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Example: `be+r`
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### `*`
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Expression matches zero or more.
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Example: `be*r`
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### `{}`
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Expression matches within specified ranges (matches this many times):
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- Match exactly: `{4}`
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- Match minimum: `{4,}`
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- Match between: `{4,9}`
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Example: `be{1,2}r`
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### `?`
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Makes the expression optional or lazy.
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Example: `colou?r`
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### `|`
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Works like OR. It waits for one of the expressions it reserved to match.
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Example: `(c|r)at`
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## Common Regular Expressions
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- IPv4-Address: `\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}`
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- MAC-Address: `(?:[0-9a-fA-F]{2}\:){5}[0-9a-fA-F]{2}`
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- Hex Color Codes: `^#?([a-fA-F0-9]{6})$`
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- Mail Address: `^([a-zA-Z0–9._%-]+@[a-zA-Z0–9.-]+\.[a-zA-Z]{2,6})*$`
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