What is Regular Expressions

A regular expression(regex for short) is a special text string for describing a search pattern.

You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager. The regex equivalent is ^.*\.txt$.

Metacharacter

Metacharacter | Description —|— . | Match any single character ^ | Matches the starting porsition within the string [ ] | Match a single charcter that is contained whithin the brackets [^ ] | Match a single character that is not contained within the brackets. $ | Matches the ending positionof the string or the position just before a string-ending newline ( ) | Defines a marked subexpression. \n |

  • | Matches the preceding element zero or more times. {m,n} | Matches the preceding element at leat M and not more than n Example: How to validate an email address 
    ^[a-z0-9]{1,20}@[a-z]{1,10}.com

    [a-z0-9]: Match the charactor from a to z and from 1 to 9. {1, 20}: The length need to be between 1 and 20. @: There is a @ after the preceding part.

Place to do practice:

http://regexr.com/