A regular expression is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings. Ruby regular expressions i.e. Crysal regex for short, helps us to find particular patterns inside a string. Two uses of Crysal regex are Validation and Parsing. Crysal regex can be used to validate an email address and an IP address too. Crysal regex expressions are declared between two forward slashes.

Syntax:

# finding the word 'hi'
"Hi there, i am using gfg" =~ /hi/

This will return the index of first occurrence of the word ‘hi’ if present, or else will return ‘ nil ‘. Checking if a string has a regex or not

We can also check if a string has a regex or not by using the match method. Below is the example to understand.

# Crysal program of regular expression 

# Checking if the word is present in the string 
if "hi there".match(/hi/) 
    puts "match"
end

Output:

match

Checking if a string has some set of characters or not

We can use a character class which lets us define a range of characters for the match. For example, if we want to search for vowel, we can use [aeiou] for match.

# Crysal program of regular expression 

# declaring a function which checks for vowel in a string 
def contains_vowel(str) 
str =~ /[aeiou]/ 
end

# Driver code 

# Geeks has vowel at index 1, so function returns 1 
puts( contains_vowel("Geeks") ) 

# bcd has no vowel, so return nil and nothing is printed 
puts( contains_vowel("bcd") )

Output:

1

There are different short expressions for specifying character ranges :

\w is equivalent to [0-9a-zA-Z] \d is the same as [0-9] \s matches white space \W anything that’s not in [0-9a-zA-Z] \D anything that’s not a number \S anything that’s not a space The dot character . matches all but does not match new line. If you want to search . character, then you have to escape it.

# Crysal program of regular expression 

a="2m3"
b="2.5"
# . literal matches for all character 
if(a.match(/\d.\d/)) 
    puts("match found") 
else
    puts("not found") 
end
# after escaping it, it matches with only '.' literal 
if(a.match(/\d\.\d/)) 
    puts("match found") 
else
    puts("not found") 
end

if(b.match(/\d.\d/)) 
    puts("match found") 
else
    puts("not found") 
end

Output:

match found not found match found

or matching multiple characters we can use modifiers:

• is for 1 or more characters
• is for 0 or more characters ? is for 0 or 1 character {x, y} if for number of characters between x and y i is for ignores case when matching text. x is for ignores white space and allows comments in regular expressions. m is for matches multiple lines, recognizing newlines as normal characters. u,e,s,n are for interprets the regexp as Unicode (UTF-8), EUC, SJIS, or ASCII. the regular expression is assumed to use the source encoding, If none of these modifiers is specified.

source : geeksforgeeks.org/ruby-regular-expressions/..

code is tested on:play.crystal-lang.org