Regular Expressions (RegEx or RegExp)
A Regular expression (sometimes called a rational expression) is a string that describes a search pattern used when working with text. Regular expressions can be used with grep (the global regular expression parser) and sed (stream editor) and also used in shell scripting , Perl, Java, python and so on.
A Regular Expression contains one or more of the following:
|
(.) dot |
It will matches any single character and equivalent to ? (question mark) in standard wildcard expressions. |
|
(backslash) |
backslash is the escape character i.e. to protect a subsequent special character. Thus, “\” searches for a backslash. Note you may need to use quotation marks and backslash(es). |
|
(.*) (dot and asterisk) |
Used to match any string and equivalent to * in standard wildcards. |
|
* (asterisk) |
The preceding item will be matched zero or more times. |
|
{N} |
The preceding item is matched exactly N times. |
|
{N,} |
The preceding item is matched N or more times. |
|
{N,M} |
The preceding item is matched at least N times, but not more than M times. |
|
^ (caret) |
It means “the beginning of the line”. So “^a” means find a line starting with an “a”. |
|
$ (dollar sign) |
It means the end of the line”. So “a$” means find a line ending with an “a”. |
|
– (hyphen) |
Used to represent range. |
|
| (pipe ) |
This makes a logical OR relationship between wildcards. |
