A regular expression is the set of rules that define a pattern. A pattern is expressed between the delimiters / ... /
The String method search() returns the position of the first occurence of the pattern if it exists or -1 if it does not.
In its simplest form a pattern could be a string of characters: /abc/. str.search(/abc/) will return the position of the first occurrence of the string "abc" in str or -1 if the pattern does not exist. If you want the pattern to be case insensitive the add the character i at the end, like so /abc/i
The following are meta characters in a regular expression.
. * + ? ^ $ [ ] ( ) { } | \
To escape the special significance of these characters in a regular
expression the back slash (\) is used.
The period . is a placeholder for a single character other than the new line character.
To denote a range the square brackets are used. For example [aeiou] will match any vowel character. The hyphen is used as a shorthand like [a-z] which represents any lower case letter. The caret (^) indicates not in the range. [^0-9] will match any character that is not a digit.
The following escape sequences denote pre-defined ranges:
| \d | Any digit [0 - 9] |
| \D | Not a digit [^0 - 9] |
| \w | Any alphanumeric character and the underscore character |
| \W | Not an alphanumeric character |
| \s | White space, tab, or newline |
| \S | Not a white space, tab, or newline character |
One can denote the position of a pattern within a string. The caret (^) denotes the beginning of the string. The pattern /^abc/ will match a string starting with abc. The dollar sign ($) denotes the end of the string. The pattern /xyz$/ will match a string that ends with xyz. \b denotes word boundary and \B denotes not a word boundary.
There are several ways to denote a recurring pattern:
The parenthesis () is used to group patterns. To indicate a choice of patterns we can write something like - /(abc)|(def)|(ghi)/. This will match a pattern with abc or def or ghi.
There are several functions in the String class that use regular expressions as input parameter. For a given String s here is how you would use a regular expression regex in these methods:
| s.match (regex) | Returns the part of the string that matches the regular expression |
| s.split (regex) | Returns an array of substrings of s divided at every occurence of the regular expression. The regular expression is not included as a part of the substrings |
| s.search (regex) | Returns the position of the first match the regular expression and -1 if there is no match. |
Here is an online Regular Expression tester that you can use to try out your regular expression skills: