regexp.rst

Regular expressions

By the help of regular expressions (regepx) diverse text patterns can be defined. Regexps are used by the most GNU text processing utilities (e.g. grep, gawk, sed) and text editors (vi, emacs). Here is a very short and incomplete list of special regexp characters:

.         (dot) any character except new line
^         beginning of the line
$         end of the line
[abc]     one character from the set in the brackets
[^abc]    one character not in the set in brackets
[a-z]     one character from the interval in brackets (inclusive)
[^a-z]    one character not in the set in brackets
( )       make group in pattern
{min,max} repetition of the previous character or group, max part is optional
p1 | p2   p1 pattern or p2 pattern
p *       any number of repetition of p pattern, including zero equivalent to p{0,}
p+        one or more repetition of p pattern, equivalent to p{1,}
p?        zero or one repetition of p pattern, equivalent to p{0,1}
\         escape the special meaning of the next character (e.g. \. the dot character, not any character)

Example regular expressions

contains the word "apple"  
regexp: apple
matching text: an apple on the table

begins with the word "apple"
regexp: ^apple
matching text: apple on the table

exactly the world "apple"
regexp: ^apple$
matching text: apple

begins with the word "apple" and any number of spaces after
regexp: ^apple \*$
matching text: apple    

begins with character "a" and contains 5 characters
regexp: ^a....$ or ^a.{4}$
matching text: apple

begins with character "a" or "A" and contains 5 characters
regexp: ^[aA].{4}$
matching text: Apple

there is a letter in the text
regexp: [a-z]|[A-Z]
matching text: ApPlE

integer number with sign regexp: ^[-+]?[0-9]+$ matching text: -0000123

integer number, no leading zeros regexp: ^0|([-+]?[1-9][0-9]*)$ matching text: 123

float number with exponetial part (e.g. -1.234e-7) regexp: ^-?[0-9]+.?[0-9]([eE][+-]?[0-9])?$ matching text: -1.234E-5

angle in DMS (e.g. 23-34-45) regexp: ^[0-9]([0-9]){0,2}(-[0-5][0-9]?){0,2}$ matching text: 102-92-31

valid email address regexp: ^[_a-z0-9-]+(.[_a-z0-9-]+)@[a-z0-9-]+(.[a-z0-9-]+)$ matching text: siki.zoltan@epito.bme.hu

line starts with adam or eva regexp: ^(adam|eva) matching text: adam and eva in the paradise matching text: eva and adam in the paradise