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diff --git a/absl/strings/internal/str_format/parser.h b/absl/strings/internal/str_format/parser.h
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+++ b/absl/strings/internal/str_format/parser.h
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+#ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_
+#define ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_
+
+#include <limits.h>
+#include <stddef.h>
+#include <stdlib.h>
+
+#include <cassert>
+#include <initializer_list>
+#include <iosfwd>
+#include <iterator>
+#include <memory>
+#include <vector>
+
+#include "absl/strings/internal/str_format/checker.h"
+#include "absl/strings/internal/str_format/extension.h"
+
+namespace absl {
+inline namespace lts_2018_12_18 {
+namespace str_format_internal {
+
+// The analyzed properties of a single specified conversion.
+struct UnboundConversion {
+  UnboundConversion()
+      : flags() /* This is required to zero all the fields of flags. */ {
+    flags.basic = true;
+  }
+
+  class InputValue {
+   public:
+    void set_value(int value) {
+      assert(value >= 0);
+      value_ = value;
+    }
+    int value() const { return value_; }
+
+    // Marks the value as "from arg". aka the '*' format.
+    // Requires `value >= 1`.
+    // When set, is_from_arg() return true and get_from_arg() returns the
+    // original value.
+    // `value()`'s return value is unspecfied in this state.
+    void set_from_arg(int value) {
+      assert(value > 0);
+      value_ = -value - 1;
+    }
+    bool is_from_arg() const { return value_ < -1; }
+    int get_from_arg() const {
+      assert(is_from_arg());
+      return -value_ - 1;
+    }
+
+   private:
+    int value_ = -1;
+  };
+
+  // No need to initialize. It will always be set in the parser.
+  int arg_position;
+
+  InputValue width;
+  InputValue precision;
+
+  Flags flags;
+  LengthMod length_mod;
+  ConversionChar conv;
+};
+
+// Consume conversion spec prefix (not including '%') of '*src' if valid.
+// Examples of valid specs would be e.g.: "s", "d", "-12.6f".
+// If valid, the front of src is advanced such that src becomes the
+// part following the conversion spec, and the spec part is broken down and
+// returned in 'conv'.
+// If invalid, returns false and leaves 'src' unmodified.
+// For example:
+//   Given "d9", returns "d", and leaves src="9",
+//   Given "!", returns "" and leaves src="!".
+bool ConsumeUnboundConversion(string_view* src, UnboundConversion* conv,
+                              int* next_arg);
+
+// Parse the format string provided in 'src' and pass the identified items into
+// 'consumer'.
+// Text runs will be passed by calling
+//   Consumer::Append(string_view);
+// ConversionItems will be passed by calling
+//   Consumer::ConvertOne(UnboundConversion, string_view);
+// In the case of ConvertOne, the string_view that is passed is the
+// portion of the format string corresponding to the conversion, not including
+// the leading %. On success, it returns true. On failure, it stops and returns
+// false.
+template <typename Consumer>
+bool ParseFormatString(string_view src, Consumer consumer) {
+  int next_arg = 0;
+  while (!src.empty()) {
+    const char* percent =
+        static_cast<const char*>(memchr(src.data(), '%', src.size()));
+    if (!percent) {
+      // We found the last substring.
+      return consumer.Append(src);
+    }
+    // We found a percent, so push the text run then process the percent.
+    size_t percent_loc = percent - src.data();
+    if (!consumer.Append(string_view(src.data(), percent_loc))) return false;
+    if (percent + 1 >= src.data() + src.size()) return false;
+
+    UnboundConversion conv;
+
+    switch (percent[1]) {
+      case '%':
+        if (!consumer.Append("%")) return false;
+        src.remove_prefix(percent_loc + 2);
+        continue;
+
+#define PARSER_CASE(ch)                                     \
+  case #ch[0]:                                              \
+    src.remove_prefix(percent_loc + 2);                     \
+    conv.conv = ConversionChar::FromId(ConversionChar::ch); \
+    conv.arg_position = ++next_arg;                         \
+    break;
+        ABSL_CONVERSION_CHARS_EXPAND_(PARSER_CASE, );
+#undef PARSER_CASE
+
+      default:
+        src.remove_prefix(percent_loc + 1);
+        if (!ConsumeUnboundConversion(&src, &conv, &next_arg)) return false;
+        break;
+    }
+    if (next_arg == 0) {
+      // This indicates an error in the format std::string.
+      // The only way to get next_arg == 0 is to have a positional argument
+      // first which sets next_arg to -1 and then a non-positional argument
+      // which does ++next_arg.
+      // Checking here seems to be the cheapeast place to do it.
+      return false;
+    }
+    if (!consumer.ConvertOne(
+            conv, string_view(percent + 1, src.data() - (percent + 1)))) {
+      return false;
+    }
+  }
+  return true;
+}
+
+// Always returns true, or fails to compile in a constexpr context if s does not
+// point to a constexpr char array.
+constexpr bool EnsureConstexpr(string_view s) {
+  return s.empty() || s[0] == s[0];
+}
+
+class ParsedFormatBase {
+ public:
+  explicit ParsedFormatBase(string_view format, bool allow_ignored,
+                            std::initializer_list<Conv> convs);
+
+  ParsedFormatBase(const ParsedFormatBase& other) { *this = other; }
+
+  ParsedFormatBase(ParsedFormatBase&& other) { *this = std::move(other); }
+
+  ParsedFormatBase& operator=(const ParsedFormatBase& other) {
+    if (this == &other) return *this;
+    has_error_ = other.has_error_;
+    items_ = other.items_;
+    size_t text_size = items_.empty() ? 0 : items_.back().text_end;
+    data_.reset(new char[text_size]);
+    memcpy(data_.get(), other.data_.get(), text_size);
+    return *this;
+  }
+
+  ParsedFormatBase& operator=(ParsedFormatBase&& other) {
+    if (this == &other) return *this;
+    has_error_ = other.has_error_;
+    data_ = std::move(other.data_);
+    items_ = std::move(other.items_);
+    // Reset the vector to make sure the invariants hold.
+    other.items_.clear();
+    return *this;
+  }
+
+  template <typename Consumer>
+  bool ProcessFormat(Consumer consumer) const {
+    const char* const base = data_.get();
+    string_view text(base, 0);
+    for (const auto& item : items_) {
+      const char* const end = text.data() + text.size();
+      text = string_view(end, (base + item.text_end) - end);
+      if (item.is_conversion) {
+        if (!consumer.ConvertOne(item.conv, text)) return false;
+      } else {
+        if (!consumer.Append(text)) return false;
+      }
+    }
+    return !has_error_;
+  }
+
+  bool has_error() const { return has_error_; }
+
+ private:
+  // Returns whether the conversions match and if !allow_ignored it verifies
+  // that all conversions are used by the format.
+  bool MatchesConversions(bool allow_ignored,
+                          std::initializer_list<Conv> convs) const;
+
+  struct ParsedFormatConsumer;
+
+  struct ConversionItem {
+    bool is_conversion;
+    // Points to the past-the-end location of this element in the data_ array.
+    size_t text_end;
+    UnboundConversion conv;
+  };
+
+  bool has_error_;
+  std::unique_ptr<char[]> data_;
+  std::vector<ConversionItem> items_;
+};
+
+
+// A value type representing a preparsed format.  These can be created, copied
+// around, and reused to speed up formatting loops.
+// The user must specify through the template arguments the conversion
+// characters used in the format. This will be checked at compile time.
+//
+// This class uses Conv enum values to specify each argument.
+// This allows for more flexibility as you can specify multiple possible
+// conversion characters for each argument.
+// ParsedFormat<char...> is a simplified alias for when the user only
+// needs to specify a single conversion character for each argument.
+//
+// Example:
+//   // Extended format supports multiple characters per argument:
+//   using MyFormat = ExtendedParsedFormat<Conv::d | Conv::x>;
+//   MyFormat GetFormat(bool use_hex) {
+//     if (use_hex) return MyFormat("foo %x bar");
+//     return MyFormat("foo %d bar");
+//   }
+//   // 'format' can be used with any value that supports 'd' and 'x',
+//   // like `int`.
+//   auto format = GetFormat(use_hex);
+//   value = StringF(format, i);
+//
+// This class also supports runtime format checking with the ::New() and
+// ::NewAllowIgnored() factory functions.
+// This is the only API that allows the user to pass a runtime specified format
+// string. These factory functions will return NULL if the format does not match
+// the conversions requested by the user.
+template <str_format_internal::Conv... C>
+class ExtendedParsedFormat : public str_format_internal::ParsedFormatBase {
+ public:
+  explicit ExtendedParsedFormat(string_view format)
+#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+      __attribute__((
+          enable_if(str_format_internal::EnsureConstexpr(format),
+                    "Format std::string is not constexpr."),
+          enable_if(str_format_internal::ValidFormatImpl<C...>(format),
+                    "Format specified does not match the template arguments.")))
+#endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+      : ExtendedParsedFormat(format, false) {
+  }
+
+  // ExtendedParsedFormat factory function.
+  // The user still has to specify the conversion characters, but they will not
+  // be checked at compile time. Instead, it will be checked at runtime.
+  // This delays the checking to runtime, but allows the user to pass
+  // dynamically sourced formats.
+  // It returns NULL if the format does not match the conversion characters.
+  // The user is responsible for checking the return value before using it.
+  //
+  // The 'New' variant will check that all the specified arguments are being
+  // consumed by the format and return NULL if any argument is being ignored.
+  // The 'NewAllowIgnored' variant will not verify this and will allow formats
+  // that ignore arguments.
+  static std::unique_ptr<ExtendedParsedFormat> New(string_view format) {
+    return New(format, false);
+  }
+  static std::unique_ptr<ExtendedParsedFormat> NewAllowIgnored(
+      string_view format) {
+    return New(format, true);
+  }
+
+ private:
+  static std::unique_ptr<ExtendedParsedFormat> New(string_view format,
+                                                   bool allow_ignored) {
+    std::unique_ptr<ExtendedParsedFormat> conv(
+        new ExtendedParsedFormat(format, allow_ignored));
+    if (conv->has_error()) return nullptr;
+    return conv;
+  }
+
+  ExtendedParsedFormat(string_view s, bool allow_ignored)
+      : ParsedFormatBase(s, allow_ignored, {C...}) {}
+};
+}  // namespace str_format_internal
+}  // inline namespace lts_2018_12_18
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_