A regular expression (aka regex) defines a search pattern to be applied to strings.
A search pattern can be any of the following:
A pattern defined by the regex may match:
Regular expressions can be used to:
The pattern defined by the regex is applied on the text from left to right [Vogella].
Here are some regex examples:
| Regex | Matches |
|---|---|
this is text |
Matches exactly "this is text" |
| this\s+is\s+text | Matches the word "this" followed by one or more (+) whitespace characters (\s) followed by the word "is" followed by one or more whitespace characters followed by the word "text". |
^\d+(\.\d+)? |
^ defines that the pattern must start at beginning of a new line. \d+ matches one or more digits. The ? makes the statement in brackets optional. \. matches ".", parentheses are used for grouping. Matches for example "5", "1.5" and "2.21". |
| Regular Expression | Description |
|---|---|
. |
Matches any character. |
^regex |
Finds regex that must match at the beginning of the line. |
regex$ |
Finds regex that must match at the end of the line. |
[abc] |
Set definition, can match the letter a or b or c. |
[abc][vz] |
Set definition, can match a or b or c followed by either v or z. |
[^abc] |
When a caret appears as the first character inside square brackets, it negates the pattern. This pattern matches any character except a or b or c. |
[a-d1-7] |
Ranges: matches a letter between a and d and digits from 1 to 7. (This does not not match, e.g., d1. It would however match d and 1 seperartely.) |
X|Z |
Finds X or Z. |
XZ |
Finds X directly followed by Z. |
$ |
Checks if a line end follows. |
The following meta characters have a pre-defined meaning and make certain common patterns easier to use, e.g., \d instead of [0-9].
| Regular Expression | Description |
|---|---|
\d |
any digit, short for [0-9] |
\D |
a non-digit, short for [^0-9] |
\s |
a whitespace character, short for [ \t\n\x0b\r\f] |
\S |
a non-whitespace character, short for [^ \t\n\x0b\r\f] |
\w |
a word character, short for [a-zA-Z_0-9] |
\W |
a non-word character [^\w] |
\S+ |
several non-whitespace characters |
\b |
asserts a word boundary where a word character is [a-zA-Z0-9_] |
The regex \bcat\b would therefore match cat in a black cat, but it wouldn't match it in catatonic, tomcat or certificate.
Removing one of the boundaries, \bcat would match cat in catfish, and cat\b would match cat in tomcat, but not vice-versa.
Both, of course, would match cat on its own.
Meta characters are given with the first letter of what they represent, e.g., digit, space, word, and boundary.
Uppercase symbols define the opposite.
A quantifier defines how often an element can occur.
| RegExp | Description | Examples |
|---|---|---|
{X} |
occurs X number of times, {} |
\d{3} searches for three digits, .{10} for any character sequence of length 10. |
{X,Y} |
occurs between X and Y number of times | \d{1,4} means \d must occur at least once and max four times. |
* |
occurs zero or more times; it is short for {0,} |
X* finds no or several letter X, .* finds any character sequence |
+ |
occurs once or more number of times; it is short for {1,} |
X+ Finds one or several letter X |
? |
Does not occur or occurs once; it is short for {0,1} |
X? finds no or exactly one letter X |
*? |
? after a quantifier makes it a lazy/reluctant quantifier. It tries to find the smallest match. This makes the regular expression stop at the first match. |
My email address is john.doe@unibz.it as the text to be searched."a"."d"."dd"."i" followed by either an "s" or a "t".