Regular Expression

Lookahead

• q(?=u)  : quit
• q(?!u)  : qtip  not quit

LookBehind

• (?<=a)b  :  abc
• (?<!a)b  :  xbc not ab

• ^ matches at the start of the string
• $ matches at the end of the string
• \d matches a single character that is a digit
• \w matches a "word character" (alphanumeric characters plus underscore)
• \s matches a whitespace character
• The dot matches a single character, except line break characters. It is short for [^\n] (UNIX regex flavors) or[^\r\n] (Windows regex flavors).gr.y matches gray, grey, gr%y, etc.
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