Explanation: Matches any Gmail address

• .* matches any character (except newline) zero or more times
• \@ matches the "@" symbol literally
• gmail\.com matches "gmail.com" literally (the dot is escaped)

Explanation: Matches lowercase letters followed by digits

• [a-z]+ matches one or more lowercase letters
• [0-9]+ matches one or more digits

Explanation: Matches any text containing "blog"

• .* at the start matches any characters before "blog"
• blog matches the word "blog" literally
• .* at the end matches any characters after "blog"

Explanation: Matches any ".com" URL

• .* matches any character sequence before ".com"
• \.com matches ".com" literally (the dot is escaped)

Explanation: Matches strings starting with "api."

• ^ asserts position at start of the string
• api\. matches "api." literally (the dot is escaped)

Explanation: Matches any 4-digit number

• \d matches any digit (equivalent to [0-9])
• {4} matches exactly 4 occurrences of the preceding element