Returns the zero-based index of the first occurrence of a specified lookup regular expression within the input string.

See indexof().



Learn more about syntax conventions.


Name Type Required Description
string string The source string to search.
match string The regular expression lookup string.
start int The search start position. A negative value will offset the starting search position from the end of the string by this many steps: abs(start).
length int The number of character positions to examine. A value of -1 means unlimited length.
occurrence int The number of the occurrence. The default is 1.


The zero-based index position of match.

  • Returns -1 if match isn't found in string.
  • Returns null if:
    • start is less than 0.
    • occurrence is less than 0.
    • length is less than -1.


  • Overlapping matches lookup aren't supported.
  • Regular expression strings may contain characters that require either escaping or using @'' string-literals.


    idx1 = indexof_regex("abcabc", @"a.c"), // lookup found in input string
    idx2 = indexof_regex("abcabcdefg", @"a.c", 0, 9, 2),  // lookup found in input string
    idx3 = indexof_regex("abcabc", @"a.c", 1, -1, 2),  // there's no second occurrence in the search range
    idx4 = indexof_regex("ababaa", @"a.a", 0, -1, 2), // Matches don't overlap so full lookup can't be found 
    idx5 = indexof_regex("abcabc", @"a|ab", -1)  // invalid start argument


idx1 idx2 idx3 idx4 idx5
0 3 -1 -1