Regular expression patterns explained: a.b, a*b, and a.*b

[Rajeev Bagra]

Source: Created with help of AI tool

Here’s a detailed comparison of the regular expressions a.b, a*b, and a.*b:

1. a.b

• Meaning: Matches a string where there is an ‘a’ followed by exactly one character (any character) and then followed by ‘b’.
• Pattern Breakdown:
• a: Matches the character ‘a’.
• .: Matches exactly one character (any character except newline).
• b: Matches the character ‘b’.
• Example Matches:
• “aXb” (where X is any single character)
• Example Non-Matches:
• “ab” (missing a character between ‘a’ and ‘b’)
• “aXXb” (too many characters between ‘a’ and ‘b’)

2. a*b

• Meaning: Matches a string where there are zero or more ‘a’ characters followed by a ‘b’.
• Pattern Breakdown:
• a*: Matches zero or more occurrences of ‘a’.
• b: Matches the character ‘b’.
• Example Matches:
• “b” (zero ‘a’s followed by ‘b’)
• “ab” (one ‘a’ followed by ‘b’)
• “aaab” (three ‘a’s followed by ‘b’)
• Example Non-Matches:
• “a” (missing ‘b’)

3. a.*b

• Meaning: Matches a string where there is an ‘a’ followed by zero or more characters of any kind and ending with ‘b’.
• Pattern Breakdown:
• a: Matches the character ‘a’.
• .*: Matches zero or more occurrences of any character (except newline).
• b: Matches the character ‘b’.
• Example Matches:
• “ab” (zero characters between ‘a’ and ‘b’)
• “acb” (one character ‘c’ between ‘a’ and ‘b’)
• “aXYZb” (multiple characters ‘XYZ’ between ‘a’ and ‘b’)
• Example Non-Matches:
• “a” (missing ‘b’)

Summary

• a.b: Matches ‘a’ followed by exactly one character and then ‘b’.
• a*b: Matches zero or more ‘a’s followed by ‘b’.
• a.*b: Matches ‘a’ followed by zero or more characters of any kind and ending with ‘b’.

[Author]

Posts