sinairv/RegexRecipes.md

## RegexRecipes.md

      
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              RegexRecipes.md
            
          
    Regular Expression Recipes

Identical character repetition detection

Match 2 repetitions of a character
(.)\1

Match 3 repetitions of a character
(.)\1{2}

Match 3 or more repetitions of a character
(.)\1{2,}

Note: \1 is referring to to group matched just before it.
Matching beginning, and ending characters of input, or every line of input

Regex Anchors are used for this purpose.
\A matches the beginning of the whole input string (the position, not a character). It does not match beginning of every line of input.
^ in single line mode matches the beginning of the whole input; while in multi-line mode matches the beginning of every line of input. So it's behaviour is dependent upon how the regex engine has been instantiated.
\Z matches the end of the whole input string (again the position, not a character). It does not match end of every line of input. Matches the final line break only if the string ends with a single line break.
\z quite similar to \Z except that it never matches the final line break, even if the string ends with a single one.
$ in single line mode matches the end of the whole input; while in multi-line mode matches the end of every line of input. It does not match lines that contain only line-breaks. In single line mode it behaves like \Z, i.e., it matches the final line break only if the input string ends with a single one.
Note that whenever \Z, and $ match the final line breaks, they match the actual character, not the position. Therefore .\Z and .$ patterns both match the character right before line-break.