A regular expression defines a search pattern for strings. You can use regular expressions to search, edit, and manipulate text. Regex is the abbreviation for regular expression. A search pattern can be a simple character, a fixed string, or a complex expression containing special characters describing the pattern. A pattern defined by the regex may match once, several times, or not at all for a given string.
The pattern defined by the regex is applied on the text from left to right. Once a source character is used in a match, it cannot be reused. For example, the regex aba will match ababababa only two times (aba_aba__).
By default, the comparison of text with any literal characters in a regex pattern is case sensitive, white space in a regular expression pattern is interpreted as literal white-space characters.