Examples of Regular Expressions

Time -- in the format 12:30 pm or 9:15 am or 08:20 am

• [0-9][0-9]:[0-9][0-9] (am|pm|AM|PM)
  This regular expression will match with strings such as "37:80 pm". We need to restrict the allowable digits before the : to be 12 and under and those after the colon must be 59 and under.

• [00-12]:[00-60] (am|pm)
  This regular expression does not correct the above problem. The square brackets [ ] denote character sets, i.e. equivalent literals. So for example, [00-60] is the same as [001234560] which considers the single literals 0 1 2 3 4 5 6 to be equivalent. The extra 0s are simply redundant information.

• (10|11|12|0?[1-9]):[0-5][0-9] (am|pm)
  The problem with matching the digits before and after the colon has now been corrected. This regular expression looks for two digits after the colon where the first must be 0 or 1 or 2 or 3 or 4 or 5 and the second digit may range from 0 to 9. Notice that to avoid times such at 19:20, we create four possible groups of literals to match on before the colon. That is, the pattern before the colon may be 10 or 11 or 12 or 0?[1-9]. This latter expression has an optional 0 followed by any one of the equivalent digits 1 through 9.

• (1[0-2]|0?[1-9]):[0-5][0-9]( a| p)m
  This regular expression is an alternative to the one above. They both have the problem of matching with "1:00 american" and with "312:23 pm". The regular expression below adds beginning and end of word requirements to the expression to avoid these problems.

• \\<(1[012]|0?[1-9]):[0-5][0-9] (am|pm)\\>

Dollars and cents where the cents are optional.

• \\$[0-9]*\\.[0-9]{2}
  Here we find matches with dollar figures such as "$.99" and "$0.99" and "$12.01" but unfortunately this regular expression also matches in $0006.20$ and does not find a match in "$16".

• \\$[0-9]*(\\.[0-9]{2})? This regular expression attempts to correct the problem with not matching "$16" by making the cents optional. Unfortunately it creates another problem, the regular expression matches with "$" because both the dollar and cents are now optional!

• \\$[0-9]+(\\.[0-9][0-9])?
  Now at least one digit is required for the dollar amount and the cents are optional. We still have the problem of leading zeroes and we also have the problem of no longer matching "$.25".

• \\$([1-9][0-9]*|0)(\\.[0-9]{2})?
  This regular expression does not find a match where there are leading zeroes such as in "$0006.20", but it still doesn't match "$.25".

• \\$[1-9][0-9]*(\\.[0-9]{2})?|\\$0?\\.[0-9][0-9]
  This regular expression seems to do the trick, finding matches in "$16", "$12.23", "$0.75", and "$.25" and not in other strings such as "$001". To avoid a match with "$1.234" requires an end of word metacharacter.

Double words

• (\\<[a-z]+\\> ){2}
  This regular expression will match any two words such as "two words".

• ([a-z]+) \\1
  The use of the variable \\1 requires a match with the first grouped literal ([a-z]). This expression gets closer to what we want, but it finds a match in "Blythe the great" and in "What is the theme?". To correct these, we need to indicate that each of these subexpressions needs to be a word.

• .*(\\<[a-z]+\\>) \\1.*
  Now the first subexpression must be a word, but surprisingly no such restriction is placed on the second subexpression \\1. This means that it no longer finds a match in "Blythe the great" but it does finds a match in "What is the theme?". In addition, the use of ".*" allows any characters before and after the two identical expressions, which leads to a match consisting of the entire expression from the "W" to the "?".

• (\\<[a-z]+\\>)[[:blank:]]+\\<\\1\\>
  This expression requires both subexpressions to be words.