Examples

• a - matches exactly one a
• ^ a $ - matches a line containing only a
• ^ [ab] $ - matches a line containing one a or one b
• a* - matches zero or more a's
• a{3,5} - matches 3, 4 or 5 a's
• ([ab]c) - matches 'ac' or 'bc'
• ([ab]c)|b - matches 'ac', 'bc' or 'b'
• [0-9] - matches any single numeral
• [a-z] - matches a single lower case alphal character
• [a-zA-Z] - matches any single alphabetical character
• (very)? happy days - matches 'very happy days' or 'happy days'

Always remember that, if you are in doubt of the ordering in which the regular expression is evaluated,insert brackets.