Origin of Regular Expression
Note: Regular Expression came from formal language theory in 1950s by mathematician Stephen Cole Kleene.
Regular Expression is used to express all “Regular Languages” (aka Linear Languages), basically meaning, any string from a grammar that does not generate nested structure.
By 1990s, it became understood mostly as a tool in programing languages as a way to search text or find replace text. Different programing languages may have slightly incompatible syntax and features, and their regex pattern no longer has original meaning and definition in formal language theory.