Regex: ignore case sensitivity

Question

How can I make the following regex ignore case sensitivity? It should match all the correct characters but ignore whether they are lower or uppercase.

G[a-b].*

Answer 1

Assuming you want the whole regex to ignore case, you should look for the i flag. Nearly all regex engines support it:

/G[a-b].*/i

string.match("G[a-b].*", "i")

Check the documentation for your language/platform/tool to find how the matching modes are specified.

If you want only part of the regex to be case insensitive (as my original answer presumed), then you have two options:

1. Use the (?i) and [optionally] (?-i) mode modifiers:

   (?i)G[a-b](?-i).*
   

2. Put all the variations (i.e. lowercase and uppercase) in the regex - useful if mode modifiers are not supported:

   [gG][a-bA-B].*
   

One last note: if you're dealing with Unicode characters besides ASCII, check whether or not your regex engine properly supports them.

Answer 2

Depends on implementation but I would use

(?i)G[a-b].

VARIATIONS:

(?i) case-insensitive mode ON    
(?-i) case-insensitive mode OFF

Modern regex flavors allow you to apply modifiers to only part of the regular expression. If you insert the modifier (?im) in the middle of the regex then the modifier only applies to the part of the regex to the right of the modifier. With these flavors, you can turn off modes by preceding them with a minus sign (?-i).

Description is from the page: https://www.regular-expressions.info/modifiers.html

Answer 3

regular expression for validate 'abc' ignoring case sensitive

(?i)(abc)

Answer 4

The i flag is normally used for case insensitivity. You don't give a language here, but it'll probably be something like /G[ab].*/i or /(?i)G[ab].*/.

Answer 5

Just for the sake of completeness I wanted to add the solution for regular expressions in C++ with Unicode:

std::tr1::wregex pattern(szPattern, std::tr1::regex_constants::icase);

if (std::tr1::regex_match(szString, pattern))
{
...
}

Answer 6

As I discovered from this similar post (ignorecase in AWK), on old versions of awk (such as on vanilla Mac OS X), you may need to use 'tolower($0) ~ /pattern/'.

IGNORECASE or (?i) or /pattern/i will either generate an error or return true for every line.

Answer 7

C#

using System.Text.RegularExpressions;
...    
Regex.Match(
    input: "Check This String",
    pattern: "Regex Pattern",
    options: RegexOptions.IgnoreCase)

specifically: options: RegexOptions.IgnoreCase

Answer 8

[gG][aAbB].* probably simples solution if the pattern is not too complicated or long.

Answer 9

Addition to the already-accepted answers:

Grep usage:

Note that for greping it is simply the addition of the -i modifier. Ex: grep -rni regular_expression to search for this 'regular_expression' 'r'ecursively, case 'i'nsensitive, showing line 'n'umbers in the result.

Also, here's a great tool for verifying regular expressions: https://regex101.com/

Ex: See the expression and Explanation in this image.

References:

• man pages (man grep)
• http://droptips.com/using-grep-and-ignoring-case-case-insensitive-grep

Answer 10

JavaScript

If you want to make it case insensitive just add i at the end of regex:

'Test'.match(/[A-Z]/gi) //Returns ["T", "e", "s", "t"]

Without i

'Test'.match(/[A-Z]/g) //Returns ["T"]

Answer 11

In JavaScript you should pass the i flag to the RegExp constructor as stated in MDN:

const regex = new RegExp('(abc)', 'i');

regex.test('ABc'); // true

Answer 12

In Java, Regex constructor has

Regex(String pattern, RegexOption option)

So to ignore cases, use

option = RegexOption.IGNORE_CASE

Answer 13

You also can lead your initial string, which you are going to check for pattern matching, to lower case. And using in your pattern lower case symbols respectively .

Answer 14

You can practice Regex In Visual Studio and Visual Studio Code using find/replace.

You need to select both Match Case and Regular Expressions for regex expressions with case. Else [A-Z] won't work.enter image description here