Introduction to Regular Expression
@(NLP)
[TOC]
导论
Q1 什么是Regular Expression？
Q2 Regular Expression 有什么用？
Q3 Regular Expression 怎么写？
Q4 R中哪有写实用的Regular Expression的包或者函数？
Definition
wiki: A regular expression, regex or regexp(sometimes called a rational expression) is, in theoretical computer science and formal language theory, a sequence of characters that define a search pattern. Usually this pattern is then used by string searching algorithms for "find" or "find and replace" operations on strings.
涉及的领域：theoretical computer science; formal language theory
内涵：a sequence of characters that define a search pattern
通常运用：used by string searching algorithms for "find" or "find and replace" operations on strings.
Basic concepts
A regular expression, often called a pattern, is an expression used to specify a set of strings required for a particular purpose.
features:
- a simple way to specify
- more concise ways to specify
- the specification is not unique
operations to construct regular expressions:
- Boolean（布尔数学体系的） "or"
A vertical bar（竖线） separates alternatives. For example,gray|greycan match "gray" or "grey".
Boolean是否只能是两者选？
alternatives表示可选择的、备用的；没有二选一的意思。
- Grouping
Parentheses（圆括号） are used to define the scope（范围） and precedence（位次） of the operators (among other uses).For example,gray|greyandgr(a|e)yare equivalent patterns which both describe the set of "gray" or "grey".
