Question

Describe the languages specified by the following regular expressions:

1. \\(_+)/

2. (\(740\)...-...)|(...-...) (The alphabet is {1,2,3,4,5,6,7,8,9,0})

Answer 1

1.The first regular expression is \\(_+)/

\ is an escaping character which is used to escape a special character.So here // means / in the string will be matched.

Inside the bracket (), _+ is present. + implies matching one or more occurrence of the previous character . Therefore, _+ means matching one or more occurence of _ (underscore ) in the string.

/ denotes the end character will be match to /.

From the above details we can conclude that the given regular expression will matches strings like,

\_/, \__/, \___/, \____/, etc.

2.The second regular expression is (\(740\)...-...)|(...-...)

\ is an escaping character which is used to escape a special character.So here \( matches ( of the string.

740 will be matched to 740 in the string

\) will matches ) in the string

. means any character except newline character will be matched .Here three dots are present.So... denotes the string will get matches to any 3 characters other than newline.

- will matches the - in the string

Therfore (\(740\)...-...) means, this regular expression matches to the string contain (740) then any 3 characters other than newline then - (hyphen) and then any 3 characters other than newline.

(...-...) means this regular expression matches the to the string contain any 3 characters other than newline and hyphen - and any 3 characters other than newline.

The given regular expression is, (\(740\)...-...)|(...-...) which means (\(740\)...-...) or (...-...)

So there is many combination of strings which matches the given regular expression. Some of examples are,

(740),78-91'

218-702

(740)13>-076

1:}-517