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technology/tools/Regex.md

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