Examples of Regular Expressions

1. Atomic regexp:
   • any non-special character matches exactly same character
     • “E” → «E»
   • a dot “.” matches any one character
     • ”.” → «E»
     • ”.” → «:»
     • ”.” → «.»
   • a set of characters matches any character from the set:
     • ”[quack!]” → «a»
     • ”[quack!]” → «!»
     • ”[a-z]” → «q» (any small letter)
     • ”[a-z]” → «z» (any small letter)
     • ”[a-fA-F0-9]” → «f» (any hexadecimal digit)
     • ”[a-fA-F0-9]” → «D» (any hexadecimal digit)
     • ”[abcdefABCDEF0-9]” → «4» (any hexadecimal digit)
   • a negative set of characters matches any character not from the set:
     • ”[^quack!]” → «r»
     • ”[^quack!]” → «#»
     • ”[^quack!]” → «A»
   • any atomic regexp followed by “*” repeater matches a continuous sequence of substrings, including empty sequence, each matched by the regexp
     • “a*” → «aaa»
     • “a*” → «``»
     • “a*” → «a»
     • ”[0-9]*” → «7»
     • ”[0-9]*” → «``»
     • ”[0-9]*” → «1231234»
     • ”.*” → any string!
   • any complex regexp enclosed by special grouping parenthesis “\(” and “\)” (see below)
2. Complex regexp
   • A sequence of atomic regexps
   • Matches a continuous sequence of substrings, each matched by corresponded atomic regexp
     • “boo” → «boo»
     • “r....e” → «riddle»
     • “r....e” → «r re e»
     • ”[0-9][0-9]*” → any non-negative integer
     • ”[A-Za-z_][A-Za-z0-9]*” → C identifier (alphanumeric sequence with «_», not started from digit)
   • grouping parenthesis can be used for repeating complex regexp:
     • ”\([A-Z][a-z]\)*” → «ReGeXp»
     • ”\([A-Z][a-z]\)*” → «``»
     • ”\([A-Z][a-z]\)*” → «Oi»
   • Implies leftmost longest rule (aka «greedy»): In successful match of complex regexp leftmost atomic regexp takes longest possible match, second leftmost atomic regexp takes longest match that possible in current condition; and so on
     • ”.*.*” → all the string leftmost, empty string next
     • ”[a-z]*[0-9]*[a-z0-9]*” → «123b0c0»
       • ”[a-z]*” → «»
       • ”[0-9]*” → «123»
       • ”[a-z0-9]*” → «b0c0»
     • ”[a-d]*[c-f]*[d-h]*” → «abcdefgh»
       • ”[a-d]*” → «abcd»
       • ”[c-f]*” → «ef»
       • ”[d-h]*” → «gh»
3. Positioning mark
   • ”^regexp” matches only substrings located at the beginning of the line
   • “regexp$” matches only substrings located at the end of line