Monday, July 7, 2008

Regular expression in Bash

Regular expressions are handy way of quickly manipulating text data or searching any substring or character in it. Almost every language supports it. For example: perl,C,php,java,etc...

Here is some quick tips on regexp with bash.

This if statement is true if string in data variable starts with numeric values.
if [[ $data =~ ^[0-9] ]]; then

^ means start of the string
[0-9] is range from 0 to 9.
[a-z] from a to z but in lower case.
[A-Z] from A to Z but in capital. as you see it's case sensitive.

if string lasts with a digit
if [[ $data =~ [0-9]$ ]]; then

if string starts with a digit and it's not more than 3 digits
if [[ $data =~ ^[0-9]{1,3} ]]; then

{1,3} means 1 character or at most 3 characters of previous declaration

this one means that string starts with lower case letter and ends again with lower case letter
any other character or numeric value fails this if statement

if [[ $data =~ ^[a-z]+*$ ]]; then

$ means end of line
+ means one or more of the preceding element

here is search and replace statements

this one searches character "a" in data variable and changes its first match with "b| character

this one is exact with previous example with one difference, it searches for all "a" characters and replace all of them with "b" character, please notice double slash.

1 comment:

Seweryn Niemiec said...

${param/pat/word} does not use regular expressions. to search and replace using regexp you have to use combination of =~ and BASH_REMATCH.