Regular Expression
A regular expression is a group of characters or symbols which is used to find a specific pattern in a text.
- a string literal is the simplest possible regular expression.
"str"would match str(sfollowed byt, followed byr). - regular expression are generally case sensitive
Meta Characters
- special characters. does not mean anything on their own.
- some meta character have different meaning when written inside square bracket
| meta char | desc | |
|---|---|---|
| . (dot) | match any single character except for linebreak | |
| [] | char class. matches any char contained between the brackets | |
| [^ ] | negated char class. matches any character NOT contained between the brackets | |
| * | matches 0 or more repetitions of the preceeding symbol | |
| + | match one or more repetitions of preceeding symbol | |
| ? | makes the preceeding symbol optional | |
| {n,m} | match at least n but not more than m repetitions of preceeding symbol | |
| (abc) | char group | |
| | | alteration. matches either the chars before or the chars after the symbol | |
| \ | escapes character | |
| ^ | beginning of the input | |
| $ | end of the input |
Things to Remember
set character range by using hyphen inside character class.
Example: /[a-z0-9]/.
a period inside a char set means a literal period
the * (star) with a . (dot) can be used to match any string of
characters. Example: .*
braces(also called quantifiers) are used to specify the number of times a character or a group of character can be repeated. Examples:
[0-9]{3}- matches exactly 3 digits.[a-z]{2,}- matches between 2 and unlimited times[A-Z]{2, 5}- matches between 3 and 5 times