RegEx Blog

^[]().*?\|+$ and sometimes -

The ^ and $ metachars. represent the start and end of a line of text

• ^: beginning of text
• $: end of text

• ^mike: will match lines that start with mike
• mike$: will match lines that end with mike
• ^mike$: will match lines that contain only the word mike
• ^$: will match empty lines

Square brackets, called character class let you matcg any one of several characters. Supposed you want to match the word gray, but also want to find it if it was spelled grey. A character class will allow you to match either:

• gr[ae]y: match g followed by r, followed by either an e or an a followed by a y.

If you use [^...] instead of [...] the result will be a regex that matches a char class that does not include the chars followed by the ^ which is interesting because the ^ has a different result then using it outside of a character class which would match at the begining of the line.

Within a character class the character - indicates a range of chars. Instead of [0123456789ABCDEF] we can write [0-9a-fA-F]. The - is only a metachar only within a character class.

The . metachar is shorthand for a char class that matches any char. It is very convenient when you want to match any char at a particular position in a string.

The | metacharacter means "or". It allows combining multiple expressions into a single one that matches any of the individual ones. The subexpressions are calles alternatives.

• Mike|Michael: this would match Mike and Michael

The ? metacharacter means optional. It is placed after a char. that is allowed, but not required, at a certain point in an expression. The question mark attaches only to the immediate preceding char.

• flavou?r: this will match every letter but has a conditional u

Like the question mark the + and * metachars. affect the number of times the preceeding char can appear in the expression. The metachar + matches one or more immediately preceding item, while * matches any number og the preceding item, including 0

• go+al: will match goal and goooooooooooooal but not gal Name quantifiers (?+*) because they influence the quantity of the item they are attached to.

The {min, max} metasequence allows you to specify the number of times a particular item can be matched by providing your own min and max.

• go{1,5}al: matches the goal string with up to 5 goals. The sequence {0,1} is the same as ?

The \ metachar. is used to escape metachars. that have special meaning so you can match in patterns. For example, if you would like to match ? or \ chars you can precede with a \, which removes their meaning \\ or \?

Most RegEx tools will allow you to capture a particular subset of an expression with parens.

• http://([^/]+)
  • begining of the RegEx matches up to http://
  • the rest of the expression matches the character up to a /: http://blog.com/blogpost1/image would only match http://blog.com