Regular Expressions
Regular expressions (referred to as “regex” for short) is a sequence of characters that describes or matches a given amount of text. For example, the sequence bob, considered as a regular expression, would match any occurance of the word “bob” inside of another text.
Regex have both literal characters and meta characters.
In bob, all three characters are literal, i.e. the *“b”* wants to match a “b”, the “o” an “o”, etc.
We might also have the regular expression:
^bob
In this case, the ^ is a meta character, i.e. it does not want to match the character “^”, but instead indicates the “beginning of a line.” In other words the regex above would find a match in:
bob goes to the park.
but would not find a match in:
jill and bob go to the park.
Here are a few common meta-characters
Single Character Metacharacters:
. any one character
\d any digit from 0 to 9
\w any word character (a-z,A-Z,0-9)
\W any non-word character
\s any whitespace character
(tab, new line, form feed, end of line, carriage return)
\S any non whitespace character
Position Metacharacters:
^ beginning of line
$ end of line
\b word boundary
\B a non word boundary
Quantifiers (refer to the character that precedes it):
? appearing once or not at all
* appearing zero or more times
+ appearing one or more times
{min,max} appearing within the specified range
